I need a brake

[Randalla]

I have a 72 240Z with stock brake components. The other day I backed out of my garage and noticed a very soft pedal. I discovered the front brake cylinder reservoir was nearly empty, so filled it back up. I had a little more pedal, but obviously air had gotten in the line. I didn't see any fluid on the floor so assumed (I know, a bad idea) that I might have a bad master cylinder and maybe it had bled into the booster. While taking the master and booster out I was looking for fluid that had gotten past the seals in the master and perhaps contaminated the booster (all was dry). Since the master had a life-time warranty, I went and got a new one. Figured it couldn't hurt. I bench bled the new master before installing, re-bled the master back into the reservoirs once everything was back together and then bled the brakes in the proper sequence. Everything seemed good, nice high pedal and the car stopped great while testing it close to home. On a longer run however (5 miles) I began smelling brake lining just before the brakes locked up, causing me to have to pull off the road to let them cool down. My wife had pulled the emergency brake on while pumping the pedal for me (I usually just leave the car in gear) which I only noticed after pulling off. I was sure after releasing the parking brake and letting things cool down that everything would be fine, which it was until driving the same distance again (5 miles) when it locked up again. So, I'm not sure at this point what's going on. It seems like the brakes get progressively tighter as the car is driven. The pedal gets very hard before the car becomes undrivable. I should also say that I removed all four wheels and inspected to see if there was any brake fluid seepage. I found one line, at the passenger side rear, which was a little damp so I tightened everything up which seemed to stop the leak. Sorry for the long post but I wanted to provide as much information as possible to help diagnose. Any suggestions would be very much appreciated.

[Zed Head]

Sounds like the rod from the pedal to the MC is adjusted too long. The hole that allows the fluid to return to the reservoir is covered, the fluid heats up and expands, causing the brakes to apply.

One possibility. It's possible to adjust the rod from under the dash without removing the MC. A wrench, a pair of pliers and some contortions.

[beermanpete]

It sounds like the push-rods are adjusted incorrectly. When there is no free-play the master does not release fully and the symptoms you describe will occur.

There are 2 push-rod adjustments to make. One is between the brake pedal and the booster. The other is between the booster and the master cylinder. When gently pushing the brake pedal by hand you should be able to detect two distinct free-plays that gets taken up as the parts start to touch. Once when the brake pedal push-rod touches the booster and another when the booster push-rod touches the master cylinder.

The factory service manual shows the procedures. If you need the manual you can get a copy at XenonS30.

[Randalla]

Guys, thanks for the quick replies. FYI, I did not change the adjustment on the rod between the pedal rod and the booster when assembling. The rod between the master and the booster on my car has no adjustment, it's a solid rod with a small piston on one end and a bullet shaped end on the other. Again, the only new variable I added was a new master. Is it possible the new master is different in some way from the one replaced?

[tlorber]

You can never assume that the old and new MC dimensions are exactly the same. That is why you need to have about 2mm of free play between the push rod and the MC plunger. I thought all of them had an adjustment (my '73 did, as did the new booster I bought that came with a "new" pushrod). You might be able to pick up some free play in the pedal adjustment inside the car, but that is not really the correct way to do it.

If you are sure there is no adjustment you might be able to grind the pushrod down a bit, round and smooth it out to create the gap.

[Zed Head]

I oversimplified, neglected the booster in between the pedal and MC. These guys are right.

You might consider loosening the nuts holding the MC to the booster and inserting some slotted shims, if you'd like to know before you take it apart again, to shorten the rod. You can get shims at most hardware stores, I know that Ace has them. You might also be able to feel if the rod is pressing the pistons in the MC when you have the nuts loose.

I've also found that there's enough flex in the lines to remove the MC from the booster without breaking the fluid lines open, if you remove the line mounting brackets and the brake check switch. Can't remember why I did it but I didn't want to lose a good bleed.

[Randalla]

Zed Head, I like your idea of loosening the bolts holding the MC to the BB a bit. Makes sense if what you guys are suggesting the problem is. Creating a little free play without taking everything down again should verify this is where the problem is. I won't get to it till later in the week but will report back and let you know if that solves the issue. Thanks again guys.

[Randalla]

New update. Today I loosened the nuts holding the master to the booster and gained about 1/4" in order to see if more travel would solve the brakes locking. After driving the car about 5-6 miles I thought that had solved the problem, however a short while later the problem returned. This time however I checked the temperature of the rotors and drums. The drums were cool, so I'm pretty sure they are not the problem. The fronts on the other hand were very hot and there was lots of black brake dust on the wheels and the drivers side rotor was heavily glazed. So check me on this...since the front brakes seem to be the culprit and the rears are cool, can I rule out the master and the booster? The front brake calipers are the Toyota truck upgrade and they have been in place and working flawlessly for more than 10 years. Now I'm wondering if a have a bad brake caliper/cylinder, or if the front/rear biasing valve is hanging up and causing this condition. I should also say that the pedal seems to gradually get harder the longer the car is driven with less and less free play.

[SteveJ]

When the car has not been driven, raise the front wheels off the ground and try to spin the front tires. Do they spin freely, or does it seem like the pads are dragging?

Have you tried pushing the calipers back?

If only one side is dragging, I would suspect a caliper. If both sides drag, I would be more suspicious of the master. Try looking through the brakes section of the FSM and see what it says about adjusting the rod on the MC. I know I had it out of adjustment on the 240 when I replaced the MC & booster years ago. I was much more careful about the measurements the last time I did it.

Also, did you ever trace the leak in the MC? Could it have been from a bad booster?

Really you haven't ruled out anything, yet.

[Randalla]

I did find the leak at the passenger side rear where the hose and steel brake line met. I took them apart, applied some teflon tape and reassembled. I've been checking this area since and everything is now dry.

[SteveJ]

Teflon tape & brakes? I don't think that's a good idea. You could introduce debris into wheel cylinders and calipers.

Edit: Interesting link - http://grassrootsmotorsports.com/forum/grm/is-teflon-tape-okay-for-use-in-a-brake-system/36652/page1/

Edited May 6, 2013 by SteveJ

[tlorber]

One thought is that after driving for a while, when things are locking up, you could try opening the bleeder at the MC for the front circuit (which is the rear reservoir on my '73) -make sure you put some protection down for the paint. If the fluid is under pressure and squirts out, you will know that at something around the master is not allowing the pressure to be released when the brake pedal is released.

Edited May 7, 2013 by tlorber