front disc brake lockup

[vargaspj]

I bought my 1952 m37 with disc brakes already installed in the front. I have replaced the rear wheel cylinders and shoes and the rear brakes work fine. 4 months ago the master cylinder was leaking so I replaced it with the same original style unit. I removed the brake vent line, made sure it was clear and replumbed it to the carb air horn (it was just open on the far end). The vent shutoff on the master cylinder is open. When I apply the brakes the front discs lock and I can't move the truck. I tried pumping the brakes to see if the discs would let go but they stayed locked on the rotors. I cracked the bleed screw on the right front caliper and a shot of brake fluid came out indicating the system was holding pressure. I have done this before when the rotors locked and heard a crack as the caliper releases the rotor. This front brake locking has happened before while I was dmriving near my house at low speed and left me stuck in a traffic lane unable to move the truck. This scares me as this is unsafe. I thought there might be an obstruction in the front brake line but like I said earlier I got a burst of fluid out of the front right caliper bleed screw. After tossing and turning all night I'm thinking the caliper slide rails may not be lubricated or may have corrosion on them and are keeping the pistons and pads from disengaging from the rotor after the brakes are applied. Any suggestions or thoughts from similar experiences will be greatly appreciated.

[SOTVEN]

HELLO FRIEND YOU SHOULD CLEAN AND LUBRICATE THE SLIDERS, BUT I DO NOT THINK IT WILL SOLVE YOUR PROBLEM. TO VERIFY THIS, WHEN YOU RELIEVED THE BRAKES FROM FLUID THROUGH THE BLLEDING SCREWS, DID THEY RELEASE? IF THEY RELEASED, THEN IT IS NOT THE SLIDERS. IN CASE I AM RIGHT, THEN DOES YOUR MC HAVE DIFFERENT CIRCUIT FOR THE FRONT AND THE REAR? IF SO, I AM AFRAID THAT THE RETURN VALVE FOR THE FRONT GETS STUCK. YOU PRESS THE PEDAL YOU HAVE BRAKES, YOU RELEASE THE PEDAL BUT THE PRESSURE REMAINS AS IF YOU STILL PRESS THE PEDAL. HENCE THE PRESSURISED FLUID IN THEM. AS ALWAYS, IT IS HARD TO MAKE A DIAGNOSIS THROUGH EMAIL, SO WAIT UNTIL OTHERS CHIP IN, AND PERHAPS THEY MIGHT POINT SOMETHING I MISSED. GOOD LUCK.

[vargaspj]

Excellent point. Original single Circuit master cylinder. Previously when they've locked I cracked a front disc bleeder screw and the front calipers released with a bang. Upon further thought something must be keeping the piston in the master cylinder from retracting after putting on the brakes. If the vent line and vent valve on the master cylinder is open what in the heck else could be giong on? This is such a simple system. Hopefully someone may have had a similar experience in the past and recognize the problem. Having a non-drivable truck is terrible.

[SOTVEN]

HMMMM.... SINGLE CIRCUIT, THAT MEANS THAT YOUR REAR BRAKES GET LOCKED UP ALSO, NOT JUST THE FRONT. THE UPSIDE IS, THAT LIKE YOU SAID IT IS A SIMPLE SYSTEM. NO BOOST, NO PROPORTIONING VALVES NO ELECTRONICS. UNLESS SOMETHING HAS GONE TERRIBLY WRONG WITH YOUR BRAKE PEDAL LINKAGE, AND THOUGH YOU PULL YOUR FOOT OF THE PEDAL, BRAKES REMAIN PRESSED WITHOUT YOU KNOWING, THEN I AM 99% SURE THAT THE PROBLEM LIES WITHIN YOUR MC. IMHO, GET A REBUILT KIT AND REDO THE MC. TAKING CHANCES WITH BRAKES IS NOT A FUN THING, ESPECIALLY WITH A 6000 LBS TRUCK. IN ANY EVENT, GOOD LUCK

[vargaspj]

Brand new original style master cylinder installed 3 months ago.....

[achiem37]

Sounds like you have too much residual pressure in the front disc lines. The original MC has a check valve to hold back around 10 psi to keep the rear wheel cylinders from retracting too far internally. You might need to remove the inner MC check valve and install a 2 psi residual pressure valve for the front discs and a 10 psi residual pressure valve for the rear drums. These can be bought at any speed shop. I would check the front flex lines and make sure that they are not plugged and holding pressure back as well. terry

[vargaspj]

Terry, is this info in the tech manuals somewhere? Super advice and many thanks! Pat

[powerwagontim]

Pat,
I had the same issue on my truck, with all drums. The pushrod on the master cylinder needed to be adjusted a tad longer. It took care of it.
Tim

[Troll]

I've seen two things cause this. If it just one or both front brakes, possibly a collapsed brake hose. There are three on the front brake circuit. It will act as a check valve. It's rare on most cars but common on Mercedes. It will release with time but over heats the brakes and wears pads faster. If it is all four brakes, try to relieve pressure at the rear and see if it releases the front. If that works, the master cylinder piston is not retuning all the way. Saw a guy replace all kinds of parts for the same reason on a F150 only to find the stop switch was adjusted to low not letting the brake pedal return all the way. Should be a little free travel in the push rod at the master.

[Monkey Man]

Remove your check valve alltogether, on discs it has no function...

Best Regards - MM

[vargaspj]

Has anyone ever taken the residual pressure valve out of the original single circuit master cylinder? How do you do it? If I can remove it from the master cylinder I can leave it out of the front line going to the disc brakes and put an incline valve (10 psi?) In the line going to the rear drum brakes.

[achiem37]

The internal MC check valve is removed during the overhaul of the MC. It is the last piece out during teardown and is a cone shaped metal stamped disc with holes in it. There is a rubber or metal disc that acts as the seat to seal in a little pressure when the brake pedal is released. I don't know if removing this check valve will effect the spring seat in the bore of the MC or not. If there is a way to remove the seat portion of the check valve then it could be returned into place and it would let the pressure drop. The wheel cylinders due need some residual pressure in them to keep the cups out against the brake shoes as the return spring is quite strong. If the wheel cylinders return back farther than normal you'll have a very soft / long pedal until they hit the shoes and drums and then can build pressure . The 10 psi check valve I put in my truck when I had the dual system MC installed improved the pedal feel as the brake shoes were now closer to the drums. This means it takes less pedal travel to push them out to contact the drum surfaces. But make sure your hoses are good and the brake pedal rod has some free play in it before it starts to push in the MC piston. These are 2 simple checks that can save some problems no matter how the system is configured. Most modern MC's use a 2 psi check valve to hold the caliper pistons lightly against the pads. Disc/Drum MC's have a proportioning valve in the circuit to due the same thing and keep some higher pressure to the drum brakes. They also give the drum brakes a head start before the calipers start to actuate to even out the braking effort. Check on line about this proportioning valve and the residual pressure valves in 2 and 10 psi values. It's good info when dealing with brake systems.

[retiredguy]

If you loosened the brake line at the caliper and they released with a bang you have a lot more than 10 psi in the line. I have seen this problem a few times and every time I saw it, it was the pustrod to the master too long. Its like keeping your foot on the pedal and not letting it come up all the way. There was the same problem on another forum where the problem was solved by adjusting the master push rod. i know you said the master was the same as the old one, but I have seen rebilders install a piston that did not have the pushrod hole deep enough.

my 2 cents......

[vargaspj]

I've read and reread all the posts and sincerely thank all the posters for their time and valued input.
The posts brought up issues and details I hadn't focused on and really helped me try and analyze what's going on.
At this point I think I'll re-adjust the brakes (pushrod and pedal adjustment) and check (or replace) all 3 of the front flex-lines to make sure they're not collapsing and trapping pressure.
After further research and talking to Wilwood Engineering it seems that a 10 PSI residual pressure for drum brakes is pretty standard and the residual pressure valve would either be in the Master Cylinder (MC) or the brake line "T". I looked up the brake line "T" and it looks like it is just a fitting; so it looks like it is that disc in the MC referenced in the posts.
Also per Wilwood no residual pressure valve is usually required for disc brakes unless the MC is mounted below the horizontal plane of the calipers and fluid drain back occurs from gravity and vibration and then a 2-pound residual pressure valve is used.
I hate to modify the original style MC so I'll try the above adjustments first.
If that doesn't work I think I'll buy a rebuild kit for the MC and modify and install the Key P - VALVE, ASSEMBLY from the MC as shown in the G741 parts manual page 243 Figure 12-4. Master Cylinder - exploded view by removing the center portion but leaving the outer lip so the Spring will seat and be at the proper length behind the piston in the MC. This should remove the residual pressure function from the MC. I can then install a Wilwood Engineering (PN 260-3279) 10-lb Residual Pressure Valve in the rear line for the drum brakes on the rear (and a Wilwood Engineering (PN 260-3278) 2-lb Residual Pressure Valve in the front line for the front disc brakes if I need it). I hate to mess with the MC but after adjusting and checking the system I'll have to give it a try because right now it's basically not drivable since I don't know when or where the front discs will lock-up.
Please post any further thoughts.
The more I work and read about these trucks on the forum it seems that they seem to all have just a little bit different personality.......
Thank you. Pat

[MSeriesRebuild]

The residual check valve is in the master cylinder. If you remove the check valve with a rear drum system, you will have to be constantly adjusting the shoe to drum clearance, or you will find yourself pumping the pedal at each brake application in order to build pressure to get a decent pedal height.

I have one more question for you that I don't believe anyone else has asked; where did you purchase this new original type master cylinder from? Once this question is answered, then we can talk further.

Residual pressure for drum brakes should be 10#. Residual pressure for disc brakes should be no more than 2#.