Front Brakes

[Ramses]

Hey,

I was hoping that you could help me here. About a month ago my power assist on my brakes went out. I switched it along with my master cylinder and everything seemed to work fine for a while. Now after about 30 minutes of riding around, the front breaks seem to lock up. I can still drive, but it takes alot more power to shift gears because the front breaks are engaging all the time. This only happens periodically and it only happens when the break pedal gets really stiff. After I park it for a bit, perhaps 20 minutes or so, the pedal relaxes and so do the front breaks. This is intermittent at best and only happens every few times that I take her out for a spin.

Any suggestions.

I was thinking that because the break lines should be replaced and the front calipers are the original ones that they should be replaced as well, but who ever heard of this happening intermittently?

Help :ermm:

[Arne]

Yup. The pushrod in the power booster is mis-adjusted, it is a touch too long for the new master cylinder. You'll need to pull the master cylinder away from the booster and adjust the tip of the pushrod. DO NOT pull the pushrod out of the booster, if you do there are parts inside the booster that can fall out of place.

[JimmyZ]

Arne you're too fast! You got the post in just before me:)

The calipers are your first suspects. How much resistance do you get when the pads are off and you have to compress a piston? If you have to use any force when using a C-clamp style compressor for the pistons then it's time for new rubbers as their elasticity causes the pads to retract a bit after the hydraulic pressure is off. I'd get a rebuild kit for them or exchange for rebuilt. If you can afford it, new is best of course. Make sure that the exchange calipers have good threads where the brake line attaches. I got a stripped caliper once.

Something might be amiss w/ the master vac. I just rebuilt mine and got to study it's inner workings. Basically it is a chamber with vaccum applied to both sides until you press the pedal. Moving the pedal causes the back side to go to atmospheric pressure which allows it to push via vaccum the master rod. It returns to neutral when vaccum is restored to the back side. Did you get the height on the M-Vac's pushrod right before the MC went back on?

A quick, simple way to rule out your master vac's valve would be unplug the vaccum line and drive in safe, non-congested area. Your braking will not be too effective with the vaccum removed but it will work. This would not indicate a pushrod misadjustment though.

It' very rare, but sometimes a flexible line ruptures internally and prevents/restricts fluid from flowing in one direction. (Like a flap valve) I'm sure that's not the case here. Worth considering though should all else fail.

Jim

[Arne]

Added note - this is not a new problem, it has come up several times before. Here's a link to one of them:

http://classiczcars.com/forums/showthread.php?t=21999

[Ramses]

I did a test after I drove her around for a while. I released the pressure on the master cyl and the brake fluid gushed out. I pulled the master cyl off the vacuum and looked at the plunger. It does not seem to be adjustable. It does however stick out from the vacuum a bit over an inch. If I seem to remember correctly (I traded in my old vacuum assist) the old one was shorter. I will take a pic if I get a chance today. Can I just replace the plunger? I do not have a small enough wrench to get around the plunger to adjust the length. Any suggestions?

[JimmyZ]

Sticks out an inch??? Oh boy! If I'm reading right you removed the MC and found that the pushrod was protruding an inch above the face of the MasterVac. There is definitely something wrong with that.

Is the brake pedal adjusted right? It might be that when you put the new master vac in that the clevis was out too far. Arne would have the right dimensions but I believe you want to have 8" between pedal and floor when at rest. Can't find my manual but I think that the pushrod should be recessed 3mm or so.

If pedal height is adjusted right and you are STILL protruding one inch there's something amiss inside the mastervac. If this is the case it would be easier to return/exchange it than take it apart.

IF you do decide to take it apart, I used a two pieces of plywood (W/ holes drilled for studs in MV) as the tool to unscrew the housing. Haynes manual has a picture of the real tool in use. Just approximate it with wood. Mark the alignment of the housing before you take it apart.

I had to make a plunger for mine out of 1" stainless steel as the sealing surfaces on the old one had rusted. (Have a lathe) The plunger's cross section is a "T" shape and it can only be removed when you take apart the MV

You'll get there!

Jim

[JimmyZ]

Just thinking... Try releasing the clevis (from brake pedal) and see if the plunger retracts to where it should be. If it was rebuilt right the plunger should have come to you properly adjusted.

[Ramses]

I am a bone head. You know the spacer between the master cylinder and the vacuum assist? Well I didn't. Apparently I turned it in with the core when I switched out the master cylinder. I was adjusting that pin so far in and it still did not work that I was thinking about removing the power assist and trying it again with an aftermarket power assist, then my friend that was visiting me asked why I removed the 1" spacer. Doh. I am such a bone head. Good thing another friend that Has several junk 240's allowed me to come by and pick one up for no charge.

[JimmyZ]

Awesome! Thanks for posting the outcome. My greatest brake fiasco was rebuilding the calipers and installing them on the wrong sides. It took me a while to figure out why the system wouldn't bleed. Swapped 'em back and all was well.

Good to hear you're back on the road.

Jim