Master cylinder anomaly

[Captain Obvious]

I don't have pics of the older style of master cylinder exploded, but this is what the later one (with the front circuit closest to the firewall) looks like:

Here are a couple other threads with pics and significant discussion about master cylinder operation:
https://www.classiczcars.com/forums/topic/66276-brakes-dragging/?&page=2
https://www.classiczcars.com/forums/topic/66168-brake-master-cylinder-identification/

 

[Zed Head]

It would be fun maybe to set up a bench testing unit to move fluid in and out of some sort of device.  Maybe some graduated cylinders so you could see the rate of flow.  I wonder if there is some sort of service bulletin out there with some words about the switching of the reservoirs.  Maybe something in Wick Humble's book or one of the others.  I don't have my books handy.

Nissan's FSM's usually have short explanations for the functions of the various components.  This is all I find for the 1972 master cylinder.

[Racer X]

2 hours ago, Zed Head said:

It would be fun maybe to set up a bench testing unit to move fluid in and out of some sort of device.  Maybe some graduated cylinders so you could see the rate of flow.  I wonder if there is some sort of service bulletin out there with some words about the switching of the reservoirs.  Maybe something in Wick Humble's book or one of the others.  I don't have my books handy.

Nissan's FSM's usually have short explanations for the functions of the various components.  This is all I find for the 1972 master cylinder.

Before I changed the race Z to separate master cylinders with a balance bar I was running a stock cylinder setup. During a meet over at Spokane Raceway Park one of the rear flex lines failed, while breaking hard at the end of the front straight (almost a mile long straight). I nearly didn’t get the car slowed down enough to avoid catastrophe, still going off the outside of the turn as the rear stepped out.

After I gathered it back up and tried to soldier on, it was clear the car was undriveable with only front brakes. The race was almost over so I continued, lifting way early and coasting through the corners.

 

[madkaw]

21 hours ago, Patcon said:

I don't know if I would dispute this but why do you think this is so?

Why do I think F goes to the front lines ?

 

[Patcon]

15 minutes ago, madkaw said:

Why do I think F goes to the front lines ?

 

Basically, yes. What do you think changes the output of one port versus the other?

Although Wilkes might have answered that...

[Racer X]

18 hours ago, madkaw said:

Why do I think F goes to the front lines ?

 

Because the F indicates front, and R indicates rear.

The master cylinder is two cylinders in tandem. One is designed to supply the proper amount of volume to the front calipers (which require significantly more volume than the rear wheel cylinders), the other designed to provide the correct volume for the rear.

Get them mixed up and the brakes won’t perform correctly.

 

[Zed Head]

Actually, the hydraulics of the system mean that both ends get the same volume for the same amount of stroke.

Might just be that the F and the R are there so that the mechanic knows where to put the reservoir.

[Patcon]

I agree with Zed. To get a different volume, one section would have to have more stroke or a larger bore. I don't see how thats possible with the design of the master cylinder. Maybe the outlets have different restrictions in them?

[Captain Obvious]

Any outlet restriction would not matter either*. The master cyl bore bore is the same diameter and the two circuits are in series. That means that at the master cylinder (and until downstream of the proportioning valve), It will be the same pressure in both circuits.

The circuit closest to the firewall won't build pressure until the circuit towards the front of the car builds pressure (or bottoms out mechanically). In fact, neither circuit will build braking pressure until they both do (or one of the two bottoms out). Maybe there's something "safer" in having the main braking circuit (the front axle) be the one that builds pressure as the second circuit..

Or maybe there is less volume required to be moved in the drum brake circuit before the shoes contact the drum so they put that one at the front of the master cylinder? If so, it would take less pedal travel before the other circuit (front disks) builds pressure.

Brake experts would know for sure. And that's not me.

 

*Except on transient changes.

Edited January 27, 2023 by Captain Obvious
more detail

[madkaw]

I don’t know guys - I’m not going to try and figure out Nissans engineering , but the larger capacity of the piston assembly in the BMC needs to go to the front brakes . 
My intention now is to get Classic Tube the correct info for when folks order lines . I made them aware that the early BMC’s are not readily available and most would be going to a later BMC - which means they need different lines than what’s in the early kit .

 

[texasz]

For the brake lines from the master cylinder and the difference from the early style to the later one (starting in '72) I think that this thread will clear things up.

 

[Zed Head]

CO mentioned valving.  I looked around and found a mention of residual pressure valves.  That could be a reason.  I think that the topic has come up in past discussions, I had forgotten about it.  You can hold pressure on drum brakes because there are return springs.  You wouldn't want that on the front.  Makes me realize that it could be a possible cause of dragging brakes, mixed-up lines.

Makes you wonder though if the quality level of a reman master cylinder is really up to the level of controlling residual pressure.    Another neat test bench factor to consider.  Or just do F and R.

"tandem master cylinder" is the search term to use.

http://www.britishv8.org/Articles/Tandem-Master-Cylinders.htm

"Another issue when selecting a tandem master cylinder is whether it comes with one or more internal residual pressure valves. On an OEM master cylinder designed for a car with rear drum brakes there will sometimes be a residual pressure valve in the rear brake circuit to maintain a small amount of pressure at the wheel cylinder seals. The amount of residual pressure varies from one design to another but typically is between 6 and 25psi. Disc brake calipers may or may not need a residual pressure valve depending on their design. A 2psi residual pressure valve is often recommended for aftermarket disc brake calipers."