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Any help or ideas would be appreciated. I have a 54 Chevy truck with disc brakes on the front and drum on the rear. It has a 79 Monte Carlo setup (brake booster, conpensation valve - proportioning valve with a clear fluid resevior). When making a sudden stop the right front wheel disc will lock as if it is coming on first. If you make a calculated or planned stop it does not do it. Also, seems like once it does it one time, it does not do it all the time. I checked the caliper, scuffed the pads, and changed both the left front and right front rubber brake hoses. That did not help. Also, adjusted both rear drum brakes. Checked the whole system for any leaks, none. Is there a way to adjust the pressure to the brakes on this type of system? Seems like the right front disc brake is braking first. Any help or suggestions would be appreciated. Thanks.

Maybe the left front's not working at all,
Scott

We thought of that but undid the brake line and gently pressed the brake pedal and fluid did come out of the line.

Just to expand the search...did you jack up the front end and do a test with everything together? Did both wheels get harder to turn as you genty applied the brakes with engine off?

Good idea...just because there's fluid/pressure, doesn't mean the caliper piston(s) is(are) moving.

cm

Check the flex lines between the frame and the caliper. If the left side flex line is collapsing internally on hard braking, it will delay the application of the brake and give you a pull or a lockup on the right. The lines are made up of several layers of rubber and fabric, and problems might not be obvious from the outside.
Jerry

How much does the vehicle sit without being used? Could be collecting moisture in the caliper.

I had a vehicle that would lock up on one side. Couldn't figure it out. Replaced the caliper and checked the lines and didn't locate a problem. Then I pumped the brakes really hard and went out to look and found one of my flexible lines was bulged. The bulge went down and the brakes worked OK again. There was just enough pressure to hold the one side tight a little longer, and the pedal would also get mushy. New flexibles all around and good to go.

I replaced both the left and right flex lines on the front with new ones. One thing I failed to mention, when we disconnected the left brake line, no fluid leaked out. But, when we pressed the brake pedal slightly, then fluid did come out. Shouldn't fluid have come out a little when disconnected. Like I said, when we pressed the brakes, then fluid came out like it was suppost to. Master cylinder??? Haven't tried jacking up the front with engine off to see if the brakes are holding on both sides. I am by myself today and guess I could use a block to hold the pedal down.

Fluid might not come out depending on where the master cylinder is. If it's under the truck or even height with the calipers, it will not gravity bleed.

If your caliper is slightly dragging, the extra heat will cause it to lock up first, jack up the front and see how the tires rotate. Also as mentioned above, don't assume the side that locks first is the problem, the other side could not be working at all. Have you done a complete system bleed? I know changing lines should have got the air out, but you never know!

The front brakes work as a single unit, same as the rears, so rule out the valves and master cylinder.
The proportioning valve applies the rear brakes first, then the fronts by a spring & valve setup, it also has a safety to block off front or rear in case of sudden pressure lose.
Side to side operation will be equally applied pressure, this then tells you one side has a problem, either air in the system, bad hose, bad caliper or wheel cylinder, dirty pads or shoes, or something sticking. OEM's designed it so it should always stop straight, even with a major problem at one wheel.

Joe

The master cylinder is located below the see thru fluid resevoir and below the power brake booster on the firewall which is high above all the calipers. We did a complete system bleed on both front left and right. When I get a helper, I will jack up and turn both front wheels with engine off to see if there is brakes equally on both sides. Seems like fluid should have dripped out of the brake line when it was disconnected at the flex line on the left side, but it did not until we applied brake pedal, then it did leak out.

I spoke with a mechanic friend of mine tonight, he said that when I disconnected the left brake line at the flex line and no brake fluid ran out, that it is not really a problem and not uncommon. He said I should bench bleed the master cylinder and maybe that would fix the problem. May try that in the future.

Sounds more like the left caliper is stuck on it's pins/slides or the caliper piston itself is stuck internally,
Scott

The master cylinder applies both side at the same time, so if it has air in it, neither side will work. Look at the calipers, is not uncommon to have one sticking, even new ones stick quite often.

I will be applying new front left and right calipers on Thursday and will advise if this was the problem.

Here is the latest. First, bled the brakes. No good. Next, replaced front left and right flex brake hoses. No good. Replaced both front calipers. No good. Next, did a bench bleed on the master cylinder. This helped a little, the brake did not lock up but the pedal was on the floor and the brakes spongy. That may be why. We bled the brakes again and then again. Still the pedal goes to the bloor and remains spongy. Guess the next step is a new master cylinder. We are not sure of the year or make. Can this be found by the number stamped on the bottom of the master cylinder or is there another way to find out.

One way to bleed a master cylinder is a lot of very short pumps, 1/4 to 1/2" stroke. If it has air in it, this get it out. Also, if the calipers are full, pushing them back in, might force the air out of the master cylinder lines, same for the rears.

I would double check the line routing and plumbing. Unless its hooked up wrong, a master cylinder will not cause it to pull one way! It has only one line out, follow that line down to where it Tee's off to the left and right, thats where you need to start looking.

To make a easy power bleeder, all you need is a extra master cylinder top and a air line. Drill a small hole in the lid, fill resevior to the top, apply about 30 psi, open each bleed valve till clear fluid runs out. Keep resevior full, and check it after each wheel is bleed.

Joe

We are pumping the pedal to remove the air. If I remember correctly, with a dual master cylinder, you have to bench bleed the master cylinder or you will never get the air out of the two cylinders and the brakes will feel spongy with air still present. I was not home when my friend said he bled the master cylinder. If he did not bench bleed it off the vehicle and get the air that is trapped, that may be why it is spongy. I will see him tomorrow. If he did do it by running the port lines back into the reservior until no air bubbles and then mount it back in the truck, then it must be a bad master cylinder with bad seals. Thats my thought because I don't know why it would have air unless 1. not bled right, or 2. bad master cylinder seal. If its a bad master cylinder, then I need to figure out what year. Casting number ????

I never bench bleed, if it's moving fluid under pressure, it's bled,
Scott

Thought I had the problem fixed but did not work the other day as it should. I replaced the flex rubber hoses on both front left and right, also new calipers left and right, new master cylinder and thought we had it fixed. Even adjusted the rear brakes. The right side locked up yesterday so guess I will have to start over again to find the problem. Fustrating.... When I had the hood and right fender repainted the other day, the painter removed the right tire and exposed the caliper and rotor. He also cleaned the engine with some kind of cleaner. The right never locked up prior to this so could it be he soaked the brake pads and that is what is causing the problem? I thought about replacing the right pads. I was told by several that the rotor would not be the problem or the compensation valve since it only affects the rear brakes. Do you think replacing the right brake pads is the next step.?????

It started after some painting was done. Like I said earlier, the painter used some sort of cleaner and cleaned the whole engine area. Whatever he used was very nasty. Underneath the engine, I have chrome oil pan, flywheel cover, etc. I had to get under there and try and clear his cleaner overspray off which was on thick and kind of greasy. Thats why I thought maybe the pads were soaking some of it up. I had new disc pads on both the left and right side. It only locks on the right side, which is the side that he had exposed. I did try riding the brake for about a half mile on and off to heat up the pads. That seemed to work at first but now has returned back to the same old problem. I believe the plumbing is okay since it has worked fine in the past prior to getting the painting done. If I change the disc pads, I will go ahead and replace both sides..

Is your m/cyl under the floor? If it is and you're running disc brakes did you add risidual valves to each circuit to prevent the fluid draining back?

Master cylinder is on the firewall. It has a compensation valve plumbed in the line under the m/c.

I keep re-reading this post, and can't help but think about the little German sports car we did brakes on. We put bigger calipers on the front, put in a bigger MC, replaced all 4 rubber hoses (independent rear), and replaced all steel lines. We also replaced the proportioning valve with a simple tee, because the new front calipers were so much bigger.
The thing is, getting this working right was all about the bleeding. All the on-line experts were adamant about this: bleed the furthest caliper first...the right rear, then left rear, then right front, and finally, left front. And have a couple quarts of brake fluid on hand. Not that the system is so big, but if you don't get ALL the air out, you'll have some wierd issues. That compensation valve is a concern...
Oh, and having someone pump the pedal works better than any piece of bleeding equipment you can buy, except one. Get some speed-bleeders, and do it yourself!
If bleeding the ENTIRE SYSTEM doesn't cure this problem, I suspect those "new" front calipers.

cm

If you suspect the front pads are contaminated, swap them side to side, if the problem follows the pads, replace them. The rotor shouldn't have been hurt with painting supplies.

Joe