Hot Rods I need brake help guys

I have a 39 chevy I put mustang 2 under it. My brakes are giving me he'll. everything is brand new. The master cylinder is floor mounted power brakes. My issue is after driving for a few miles say 2-3 my front brakes start dragging and eventually get to the point you can't turn by hand. If I crack a front bleeder they go back to normal.
I tried bleeding many times. I replaced the front residual valve thinking it was bad. Still same issue. What's next?? Rear brakes are fine. The pedal returns fine with engine NOT running but returns slow when car is running. Any ideas??? I need help. I don't know what's wrong. Thanks

 

Check the position of the brake rod. It sounds like the piston in the MC is not returning past the exit holes in the MC. You can also try driving it with the booster disconnected from the vacuum.

 

Fuzzy, are you saying I might need more slack in the brake rod? What will disconnecting the vacuum and driving it prove?

 

As stated above, the master cylinder pistons may not be fully returning, trapping fluid and causing a "run-away" disc brake. The first thing is to make sure the pedal has it's own return spring. Don't rely on the booster to bring the pedal back. Second, make sure the pedal can return farther than necessary to fully return the booster. The last thing to check, if there's still a problem, may be the booster push rod not adjusted for proper clearance with the primary master piston.

 

Or sometimes a bad collapsed brake hose. Gary

 

my Buick did this, the rod wasn't pushing all the way thru and the brake woukd start to drag and eventually lock up
.adjust the rod out.

 

Do you have check valves in your system? I had some bad overseas ones. I used Willwood, no more problems

 

Are you sure the master cylinder didn't have a residual valve already?

 

If pedal pushrod has adequate clearance to the booster and you suspect the clearance between the booster and master cylinder is too close , an easy check is to loosen the master cylinder nuts and slide in a body shim and retighten the nuts and give it a try. Easy way for a quick check,


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I work on modern cars for a living, I've seen this on a car with a bad brake booster if I'm remembering correctly. However , sometimes modern stuff can be a bit different than the old style stuff, but it's worth checking

 

Your brake rod is to long by a fraction of an inch.
Simple as that.

 

I had this condition and it was the rod between the master cylinder and the power brake unit was too long. Shorten it a little to see if that helps.

 

Everyone saying this or similar has my vote.

 

Ok. I shortened the brake rod and it was no better, same issue.
I stuck a shim between master cylinder and booster and still same issue.
I checked all lines for a kink or bent spot and found nothing wrong.
Same thing.... Drive 1 or so miles and front brakes get stuck. Stop and crack LH front bleeder and it releases pressure. Start driving and it does same thing again. WTF????

 

If you are letting out pressure at the bleeder you could have a bad hose. See if you are holding pressure at the master. Crack that line and see what happens. If no difference then go the next thing in line, check valve then hose. Good luck

 

Could I have a bad master cylinder and it's causing this problem? The master cylinder has 4 ports on it. I do have both lines on one side. On the other side I have my brake switch screwed in. It's on the front port. Would that be an issue? I don't know if I should just replace the master cylinder now or what to try next. I just want to be able to drive the car. It's frustrating. Thanks

 

Your holding residual pressure somehow. The next time it happens, crack the line at the master and see if the brakes release. If not go to the next component and crack the line until you find what's holding pressure. If you have a 10lb residual check valve to the front brakes, that could be your problem. Don't replace any parts until you find where the problem lies.

 

Is your master cylinder close to the exhaust? If it is, it could be heating the brake fluid and expanding it.

 

You didn't say if your MC was new. I'll assume that it is. HOWEVER, I had the same problem with my '57 Chevy 1/2 ton truck. It had been sitting for a while, and corrosion built up in the MC and partially clogged the return hole in the MC, so it slowly built pressure until the truck would barely move. Replaced the MC, bled the system, problem gone.

Chris

 

The master cylinder pushrod is the first thing to check.

You need a little free play when the brakes are released.

Not too much. Just a little.

 

When my '63 Chevy pickup was "late-modeled", whoever built the truck bolted the proportioning valve to the left frame rail directly across from the exhaust manifold. After a few miles of driving the heat from the exhaust would cause the brake fluid to expand and lock up the front brakes. I moved the proportioning valve to front crossmember below the radiator (stock location on late-model pickups) and that solved the problem. In other words, your brake problem is caused by exhaust heat.

 

Check this first. Disc brakes would need the 2lb valve.
Are both front wheels locked or just one? Does it matter which bleeder you crack open to release the pressure?

 

Many years ago i had the same sounding problem on a newly built car. Did just about everything already suggested except tracing the source of the problem. In ignorant desperation i removed the 2lb residual requiring making a longer line to replace the removed part. Problem disappeared and i ran the car for years. I'll never know if it was a duff part or what! I guess there was something in the master.

Chris

 

I worked on the brakes some more. I took the old residual valve which was new 3 months ago apart. I removed everything inside it and reinstalled. Still locks up front brakes.
I cracked front brake line at master cylinder this time and brakes unlocked. So is my master cylinder bad? How do I tell if the plunger from master cylinder to booster is the right length? They were bought as an assembly so I would think they are a matched set. I am still lost.

 

Do you have a return spring on the brake pedal? The weight of the pedal might be putting pressure on the system.

 

No spring. The pedal releases fine. It don't sag down or anything

 

Are you sure the MC isn't for a drum application? It may already have a residual valve built inside the MC that you can't see. Be sure you have a disc MC and be sure anything you put in line to the disc brakes is only a 2# valve.

SPark

 

I had this on my daily driver. It was a bad brake booster.

 

I had this exact issue. The proportioning valve (not residual valve) has a gizmo in it that slides back and forth. It stuck, and produced those symptoms. Somebody here can tell you how to "un-stick" it. Mine was the only unknown, found-in an-old-box part in my brake system, so I replaced it. If we're betting, that's where my money is!
Ralph

 

Most of you will disagree, but I'm going with air in the line somewhere, fluid is non compressible, if the brakes work cold, then the master is not making them drag. Air will compress, brake drag will create enough heat to activate the brakes after driving. I chased this with my quad for nearly a whole riding season, bear of a thing to bleed properly, but it was air. If the pushrod is too long, the brakes will drag all the time.