Question about S30 rear disc brakes....

[e_racer1999]

okay, i have been researching this for a few weeks and i want to know EVERYTHING before i begin. i've searched several forums and have recieved mixed answers. my big question is:

if i have a 1972 240 and i am going to do the rear disc swap with the maxima brackets and rotors, calipers, and master cylinder (15/16) from a 1979 280zx, what is the story with the proportioning valve? a few sources say that they have used the 240 one, a few recommend the one from the 280zx, and a few say that you don't need one at all.

the reason that the last one makes sense is that the front discs on the 240 are larger diameter than the 280zx ones, and hence the proportioning valve would seem overkill. anyway, if i do bypass the proportioning valve, how do i do it?

P.S. sorry about this long azz multi-part question, i just want to know what everone thinks!

-jason

[SP240Z]

You can disable the stock proportioning valve by just removing the internal parts. For street use, brake bias will be fine with this modification.

SP240Z (formally 240ZX)

[e_racer1999]

alright, thanks for the info! this guy on hybridz said that he has an adjustable screw-type PV, and i'm thinking about that later on down the road, but i am definitely going to hollow mine out

-jason

[SER240Z]

Just to clarify, could you just run the stock one without emptying it?

[waynekarnes]

the rear valve on the 240z is not a proportioning valve. it is a residual valve, retains a bit of pressure on the rear brake wheel cylinders ( rwc ). means, rear brake cylinders don't require a large amount of pressure to engage ( kinda like booster ).

a proportioning valve allows one to balance the rear brakes to the front. the front brakes need to engage way before the rears ( too much rear brake bias, and the arse end of the car is going to be coming around the front ). datsun engineers did a pretty good job of balancing the rear braking to the front brakes, by the size of the rear shoes, size of the drums, the size of the rotors, pads and the pressure exerted from the master cylinder. they did not use a proportioning valve.

again, the residual valve on the 240z is not a proportioning valve.

remember, when you brake, weight shifts forwards, rear of the car becomes lighter. less tire contact on the road, hit those brakes, too much brake at the rear, tires get loose, start skipping and sliding.

with a proportioning valve, you may change the bias. more brake on the front, less on the rear, of if you like, more on the rear less in front.

brakes !!! energy !!! by using the brakes we are converting kinetic energy into thermal energy.

oddly enough, the quicker you can get your brakes hot, the quicker you will stop. that said, the heat from braking, must be shunted off. the shoes, drums, pads, rotors, must be brought back within a range, where they can absorb more heat. when they reach a point where they can't get rid of the heat quicker than they absorbing it, the car won't stop.

what we need is a way to transfer the heat away from he braking components as quickly as possible. disc brakes do this better than drum brakes. thus, they stop a car quicker than drum brakes.

are disc brakes necessary on the rear of a 240z ? read on ...

the 240z is a fairly light car. many of the racers use just the stock braking components with a better material for the shoes and pads. they make sure that all these components are at their best. they flush the fluid after each race, clean cool brake fluid. hot brake fluid absorbs water. water compresses at a different rate than brake fluid. makes the brakes feels spongy, brakes don't work as well. if the fluid boils, big trouble, steam doesn't compress well in a brake system. water turns to steam quicker than brake fluid. change the fluid, dump the water.

back to components ... if i remember correctly, something like 70% of the braking is done by the front. this is the reason, rear discs on the 240z are really wasted as far as street performance rear brakes.

a good set of finned drums dissipate the heat better than non finned drums, an excellent cost effective upgrade. plus you get rid of some unsprung weight as well. the finned drums get rid of the heat in a hurry and get you stopped.

why not discs on the rear ??? don't need em, are over kill.

remember the part about heat exchange ? well, with the front doing the majority of the work. on the z car, there's not much heat exchanging going on in rear. the rear can only work so hard. everytime you stop, the front gets heavier and the rear lighter.

lighter in the rear, means less energy to convert into heat. less heat means less to cool. why disc brakes ? because they cool faster, thus you stop faster . but ... if the drum brakes cool as fast as the discs ( under the same conditions ), there is little or no advantage ( as far as stopping ) to using discs over drums. heat exchange is roughly the same.

analogy ... you haul a quarter ton of dirt in a half ton truck. easily gets the job done. neighbor has a 5 ton truck. can you haul your 1/4 ton of dirt in the 5 ton truck ? sure. do you need to ? no. is it over kill ? yes. does it look cooler ? sure it does.

if your rear drums in good condition, they do a very good job of turning kinetic energy into thermal energy. they are also good at transferring that thermal energy into the air and cooling back down, maintaining a balance of energy transfer that allows the 240z to stop the way it should.

on the street and for most track racing, disc brakes on the rear of a 240z are over kill, the average driver will not notice the difference in performance. most people upgrade the fronts and add the rear discs at the same time, so for them, yes they notice an improvement. i'd guess, 98% from the front and 2% from the rear.

the 280zx got rear discs, mainly as a keeping up with the Jones thing. not because it needed em.

most high performance cars have 4 wheel discs. most of the high performance cars are bloated pigs, when compared to the weight of a 240z. when braking, they still have a lot of weight on the rear wheels. with all that weight, the discs do a better job of converting kinetic energy into heat and cooling back down, than a set of drum brakes would. the 240z doesn't have a fat arse to haul down, thus doesn't need rear discs.

am i saying don't install rear discs ??? no way !!! your car, you do as you wish. ( remember SAFETY !!! do it correctly. ). does a 240z need rear discs ? no, it doesn't. will it improve your overall braking ? marginally, but you probably won't notice.

money is better spent on front brake upgrades, stainless steel brake hoses, better pads, finned aluminum drums and better shoes. remember, front brakes do most of the work. make them work better, car stops better.

that said ... i plan on discs on the rear of my 240z. why ? easier to work on, and the cool factor. i want em, bought em from a fellow member, they are in a box in the garage. one of my some day projects.

what am i running now ??? early toyota 4 runner calipers, on stock nonvented rotors. in the rear ? finned aluminum drums, with nismo racing green shoes. all new wheel cylinders, master cylinder, clean new dot 3 brake fluid.

stops pretty good. doubt that i will see any improvement, once i swap the rear drums for discs.

i have beat on a few doors before deciding how i wanted to go about this.

most of the time, you can use the original master cylinder. you need to remove a check valve from the drum brake master cylinder ( see repair manual ) if you want to use it.

you can install a proportioning valve ahead of the point where the brake line splits to each rear wheel. you can then gut the residual valves ( located at rear of the car, usually ahead of the rear axle ) or remove em.

i have been told ( and i think i am going to try this myself ), that you can remove the factory proportioning valves from the 79 thru 81 ( may be thru 83 ) 280zx and use them in place of the 240z residual valves. this would be much easier than installing and adjusting a proportioning valve.

last month, or the month before, grass roots racing magazine had a great 4 or 5 page article on brakes and the use of proportioning valves. suggest it as a read before diving into this project.

again, do we need rear discs to improve braking ... No ! the z is too light a car, to truly benefit from em, and the arse end gets really light when braking, finned aluminum drums, good brake shoes, wheel cylinders,and clean dry air free, brake fluid works just fine at stopping the z.

will upgrading the front brakes improve a z car's stopping power ? Yes indeed !!! do you need to ? No, the factory discs are almost more than one needs for the average street driven 240z.

if you have increased horsepower, by more than half ? then maybe you should think about upgrading the front. just want better brakes ?

upgrade the front before adding rear discs. that is the best performance improvement for the money spent.

want rear disc brakes ? go for it ! need em ? nope, most likely, you don't.

again spend your money anyway you want. your car, do as you wish.

please be safe, pay attention to details, do it right. want to hear from you again.

time for me to get off my soap box. sorry for being so long winded.

wayne

[260Zed]

Bloody marvellous post!

Long winded, but quality.

Cheers Wayne!

[e_racer1999]

^^ definitely good post, and i just want to say that the only $$ i plan on spending is new pads and brackets. other than that i have all of the other parts. i agree that upgrading the front is more of a priority than the back, which is why i want to do the toyota calipers conversion. anyway, thanks for the info!

-jason

[240ZX]

waynekarnes,

Please help us understand your explaination! You say the "residual valve" retains a bit of pressure on the rear brake wheel cylinders (rwc) means, rear brake cylinders don't require a large amount of pressure to engage (kinda like a booster).

If the residual valve retains a bit of pressure on the rear brake wheel cylinders, then surely you would have rear brakes that constantly drag. I don't think so! And if the rear brake don't require a large amount of pressure, then why would you use a device kinda like a booster? I'm not implicating that you do not understand the workings of the unit, just your explaination of it is in question.

Wouldn't it be safe to say that the residual valve is nothing more than a reducing valve, decreasing the amount of pressure to the rear wheel? So, in reality the "residual valve" proportions the the pressure between the front and rear wheels, even though the residual valve is not manually adjustable, but a fixed setting.

What-da-ya-think?

[waynekarnes]

the residual valve maintains a bit of pressure on the rear wheel cylinders, so that the brake shoes don't have as far to travel.

i am not exactly sure how to explain it ... it sounds wrong ... but it is correct. the shoes are not engaged, just anxiously sitting waiting on the side lines to get into the game.

here i borrowed this from a web site ... might do a better job of explaining than i did.

and that issue of Grass Roots Motor Sports (man, i can't exagerate how great a magazine it is ... browse an issue at barnes and nobles ) anyway the info on p valves is in the may 2004 issue.

here's what i borrowed ... hope this helps !!!

wayne

from the Mp brake web site ...

What does a proportioning valve do?

A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions.

What are the symptoms of a bad proportioning valve?

If your prop valve has gone bad your rear wheels will lock up easily, especially on wet surfaces.

What does a metering valve do?

A metering valve or "hold off valve" is used in the disc portion of a disc/drum system to hold off the application of the front discs slightly allowing the slower reacting rears to catch up. This provides rear stability on wet surfaces and reduces excessive pad wear.

What are the symptoms of a bad metering valve?

Your car will nose dive and the front pads will wear too fast.

What does a residual valve do?

There are two different uses for residual valves. The 10 lb valve is used to hold a residual pressure to the drum brakes to keep the shoes out close to the drums giving a higher firmer pedal. The 2 lb valve is used in the disc when the master cylinder is lower than the calipers to prevent back siphoning of the fluid from the master.

What are the symptoms of a bad residual valve?

The brakes will be very spongy and you will need to pump the pedal to get good brakes.

What is a combination valve?

A combination valve incorporates metering and proportioning into one valve providing all necessary valving for disc/drum systems.

See below

Should I use an adjustable proportioning valve?

Not if you can help it. The adjustable valve will only provide the proportioning function and not the metering that is needed.

VALVING

VALVE TYPE

DESCRIPTION

METERING

The metering or hold off valve is used in the brake system to better balance the front to rear brakes. The valve does not allow the pressure to rise at the front disc brakes until the pressure at the rear drums has risen sufficiently to overcome the brake shoe springs. At this point the valve opens to allow full pressure to build at the front brakes.

PROPORTIONING

The proportioning valve modulates the pressure to the rear brakes. The modulation is necessary to minimize rear wheel lock up found in heavy braking and to compensate for the differences in braking conditions in front disc / rear drum systems. As pressure is applied to the system full pressure is allowed to the rear drums up to a certain point. Beyond that point the pressure to the rear is reduced preventing rear brake lock up.

RESIDUAL

There are two different residual valves. A ten pound residual valve will maintain a line pressure to the rear to keep the drum brake shoes out close to the drums giving a higher firmer pedal. Without a ten pound residual pressure to the rear you will experience a spongy pedal. A two pound residual valve is required whenever the master cylinder is lower than the calipers to prevent backflow of fluid from the calipers to the master.

COMBINATION

A combination valve incorporates metering and proportioning into one valve. These are available for disc/drum or drum/drum systems.

ADJUSTABLE PROP VALVE

The adjustable proportioning valve is used when you have a special rear condition that requires higher or lower pressure than a normal condition. You should always use a metering valve to the front when using the adjustable

[gundee]

Have to disagree here. The subject valve in the rear of the Z is in fact a proportioning valve. This is clearly spelled out and in diagram as well in the FSM. The Z does not have a seperate residual pressure valve. It is not required in this case because the rear drums are the self-adjusting type on the Zcar.

[waynekarnes]

on the proportioning valve issue, i stand corrected. appears i was misinformed. had been told by factory trained mechanics that they were residual valves.

i stand by the rest of the information regarding rear drums vs. rear discs.

in the brake section of the 1973 nissan factory service manual ...

page br-2 1973 factory service manual, 9th line down ... moreover, the brake system is equipped with a proportioning valve to prevent skid due to early rear wheel locking.

page br-5, header "brake line", in addition, the rear wheel side circuit is equipped with the proportioning valve ...

in the 1973 fsm, the proportioning valve is referred to as the NP valve, it is located under or near the master brake cylinder, ahead of the distribution block ( where the brake tubes/lines split to go to front brakes and rear brakes ). see figure br-7 on page br-5

there is no mention, nor diagram in the 73 manual regarding proportioning valves at the rear of the z ...

i have not seen this NP valve on my 72 z, it has valves at the rear of the car. unfortunately, i can't seem to locate my 72 fsm.

as far as newer z cars in the 30 series , according to the 1973 factory service manual, appears that 73 and later z cars have NP valves. it works in conjunction with a warning light that comes on and warns the driver when there is an incorrect difference in pressure between the front and rear brakes. an unbalance of pressure could cause the rear brakes to lock prematurely. was this NP valve added late in the 73 year ?

the haynes manual refers to the rear valves as pressure regulating valves ... i take that to mean proportioning valves and haynes indicates that they are located at the rear of z.

according to haynes the NP valve wasn't used on the 240z, only on the 260 and 280z cars. the factory manual shows em on the 73 models. strange. i don't remember seeing one on my old 73 z either. it had valves in the rear ... my factory manual has a foreward saying, manual to assist dealers in servicing the 1973 240z. so, for sure, that and the cover of my fsm, make it a 73 manual.

i know that when i 1st started thinking of adding 280zx rear disc brakes to my last 72 240z, back around 7 or 8 years ago, i was told to remove the residual valves, as they would maintain a 10 lb pressure on the discs and the rear brakes would be seized, the 2nd time i pushed on the brake pedal. it was suggested, that i use the true proportioning valves from the 280zx, or add an inline proportioning valve ahead of the split of the rear brake lines.

according to the f s manual, the residual valve ( check valve ) is located in the master cylinder and the devices at the rear of the car are indeed a drum brake proportioning valves. i do know that if one should use the early master cylinder with rear disc brakes, the check valve that must be removed, or the rear calipers will be locked up, the 2nd time you press the brake pedal.

i bow and admit my mistake ... residual valve ( check valve ) is in the master cylinder. the rear valves are indeed drum brake proportioning valves.

i went back over some of my old info from, the been there done that people, that have done the disc brake swap. some of them refer to the rear devices as residual valves.

i didn't find any info indicating that any of them attempted to use the 240z drum brake proportioning valves. they either gutted em and used a firewall mounted proportioning valve or replaced the rear valves with those from a 280zx.

sorry for any confusion !

again, it is suggested the drum brake proportioning valves be removed when doing a rear disc conversion. to be replaced with a disc brake proportioning valves or the drum brake proportioning valves be gutted and an inline proportioning valve used. your choice, change to the zx master, or keep the 240 master if using the 240z master, remove the check valve.

in the factory manual it gives info on how to test to see if the rear proportioning valve is working. it involves a large parking lot or empty stretch of road and a few panic stops on dry pavement, from 30 mph. says rears should lock at same time as fronts or fronts a few seconds ahead of the rear. the rears should never lock before the front. this test should be done every 24,000 miles. be sure to have the manual in your z, to show the officer that is curious as to what you are doing. :-)

wayne

[gundee]

You are correct Wayne, the master/cyl takes care of any residual pressure needed.

All the 240's I have seen incl. my 72 have the pressure differential warning switch that is mounted down by the frame rail. It is just a dual action hydraulic actuated switch that turns the dash brake light on in the event the front or rear has a pressure loss. It has nothing to do with proportioning.