Front brakes locking up

[Diseazd]

The front brakes are locking up on my orange 72......I have a brand new master cylinder (Nissan). The brake lines are all new. Any ideas on the cause? I'm wondering if there is any adjustment on the push rod of the M/C? The front right wheel gets so hot you can't touch it.....the left front wheel has more than usual brake dust. Fluid level was down on the front resovoir. Any ideas.....I suck at brakes?

[yakhopper]

Sounds like you have a bad caliper, might be able to rebuild, but not to expensive to replace. Make sure you either rebuild or replace both calipers so they operate evenly.

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[Zed Head]

Similar topic just came up.  The MC rod is adjustable.  What can happen if it's too long I described in Post #15.  

 

http://www.classiczcars.com/topic/51812-brake-booster-check-valve-question/

[Diseazd]

I see where the rod should be adjusted to 8 1/8 inches from the top of the pedal to the floor. Is that adjustment when you are pushing the pedal down or with it hanging free?

[black gold man]

If you open the bleeder and the brakes are still not turning free then you know it is the caliper sticking.

[Zed Head]

I know there's a procedure in the FSM, but the one essential thing to look for is that the MC rod does not have any tension on it with the pedal up.  My method is to adjust the pedal to where to where I like it, then adjust the clevis on the rod to where the clevis pin will slide in easily without moving either the pedal or the MC rod.  That will place the MC piston at its proper position in the MC and the pedal where you want it.  You can fine tune from there to take up some small amount of play before the MC piston moves, but it won't take much before you're blocking that hole.

 

You can adjust the clevis by turning the rod, inside the MC.  It's not attached to anything, it will spin easily.  So leave the clevis over the pedal hole, and turn the rod until the clevis pin slides in.  Then maybe a bit more to take up any play.  I use a pair of pliers, or long nose vice grips to turn the rod.  It's cramped up there.   You almost can't go wrong this way, I think.

 

If your problem came from nowhere though you probably have something else going on.  Did you change something recently?

[Diseazd]

I'm going to go out and adjust the pedal rod to see if it might be applying pressure on the MC. I want to make sure that's not the case, otherwise new calipers aren't going to solve the problem. If that doesn't help, I'll install the new rotors and calipers, and if that doesn't fix things, I guess I'll check the plunger rod in the booster for adjustment (too long). Thanks everyone for your help ......I'll keep you informed as I go along.

[Zed Head]

I kind of forgot about the booster, in between.  The other way to check the rod is to loosen it until the clevis pin rattles.  Forgot that method, if it's already together.  Easier.

 

Still wondering, what initiated the problem?  You never said.

[Diseazd]

Zed Head...... Not sure what initiated the problem.....it may have been dragging for a while....just not bad. Remember...this car has been in Resto for 2 1/2 years, so I'm just getting around to driving it. Let me tell you what I did find though. The 17 mm bushing/bolt was halfway out.....the brake pedal was wobbling around, about to fall out and ruin my day! I slid it back in and bolted it down tight.....could that have been part of the problem? Next, I pulled the master cylinder off of the booster.....reaction disc stayed in place to my delight. I shortened the adjusting plunger rod about 3/16 to 1/4 inch and put it back together. I put 8 to 10 miles on it.....no brake dust and no burning smell, and no hot Panasport. You may have been right, there may have been a little pressure on the master cylinder plunger.....causing my problem. Getting dark, will put 40 or 50 miles on it tomorrow and if OK, I've got new rotors and calipers to install....they may still be a problem. Thanks for your help and everyone else's. I'll update till I'm sure it's fixed. BTW, I still can't figure out how to adjust the brake pedal ( do you take off the big spring, do you adjust it while putting light pressure against the booster etc.) I would imagine the booster plunger rod does pretty much the same thing. Thanks all ....I'll update later.

[Zed Head]

The theory on blocking the return hole to the reservoir by overextending the rod is that heat causes the brake fluid to expand, creating pressure.  Maybe you never got it warm enough.  Glad I could at least point to something.  I haven't been in to the brakes for so long that I lost track of the other rod.  I think that the one from pedal needs to be adjusted correctly so that the booster doesn't kick in early (I don't have a firm grasp on the fine details of how the booster works), and the one from the booster so that the MC piston is ready to go, with no play.  But they both are in series so maybe they both can cause the problem if mis-adjusted.

Edited July 19, 2015 by Zed Head

[Diseazd]

Put 20 miles on her this morning. It must have been caused by the 17 mm bolt (bushing that bolts the pedal to the pedal box at the top of the pedal)) that backed out letting the brake pedal wobble around, because it wasn't a problem today. If that bolt had dropped out, I think it could have been exciting . There was no heat or drag whatsoever. I guess the twisted pedal could have activated vacuum in the booster and put pressure on the brakes. Whatcha think?

[John Coffey]

If you're brake pedal mounting bolt was loose, I would stop driving the car and do a full nut and bolt check on it.  You've most likely missed tightening other fasteners.