Hot Brakes

[wxman1]

Recently converted my 69 C10 P/U to front disk brakes. The system seemed OK at first but today the rotors got very hot...twice.

Here is a pic of the master cylinder/prop valve and power booster setup.
The master cylinder is a 1971 version and the prop valve is a MBM-PV2-GM Disc/Drum Proportioning Valve. It has 2 lines in the front for each front wheel
and one slightly larger line in the rear for the rear drum shoes.

After the 2nd test drive I measured the rotor temp with a non-contact Ir thermometer and it showed 475 degrees for the left front and 460 degrees for the right front. I stuck a large screwdriver into one of the vents on the ventilated disc rotor and attempted to turn the rotor but no dice...stuck hard.
I then loosened the bleed valve on that calipher ...fluid squirted out and then the rotor could be turned.

So why is the fluid pressure on the rotor not being released? I even took the master cylinder loose after the first over-heating event and and shortened the "plunge rod". The only thing that accomplished was to change the pedal position where the brake pressure is felt. The brake pedal gets very hard.

Any help on this problem is appreciated.

Jim

[Wrenchbender Ret]

Something is not letting the MC piston go back all the way. Either the rod between the booster & MC or the stop on the pedaln arm.

[cdowns]

i wood git rid of rubba gaslines b4 truck burns to da ground

[TBONE1964]

Is the cylinder in the MC sticking? After you apply the brake and there is pressure applied to the front calipers, unbolt the master from the booster and look to see if the cylinder is stuck. You may have to try a new master cylinder.

[volksworld]

c downs is right...old rubber hoses swell shut internally...your foot has enough pressure to force fluid thru the lines but it can't return

[68gmsee]

Did you replace the front rubber brake lines? They swell up internally and allow fluid under pressure to go to the brakes but not return to the master cylinder.

Also, check the booster to master cylinder push rod to insure it's adjusted for proper clearance, lubricate all the sliding surfaces of the disc pad mounting. If all else fails, it may be the seal on the piston itself not operating properly to retract the piston. Clearances are very minimal.

[wxman1]

Ok...so more experimentation this morning. Start the truck...pump the brakes
several times. If I stick a large screw driver into one of the rotor vents and press with all my weight...it wont move. So I decide to undo the bolts on the master cylinder to power booster. master cylinder pulls away from the booster
and I can no turn the rotors. So I went in and adjusted the push rod coming out of the booster even more than before(there's no threaded area left now). Went for a couple of laps around the block and then used my IR non-contact thermometer to check the temperature of the rotors. They measure about 200-205 degrees. I'm told thats about right.

About this truck...it came with power drum brakes(69 c10) so originally I bought a 71 Master cylinder and continued to use the old power booster. I think that was my mistake. So I will order a 71 power booster and see if that makes a difference.

While the flexible portion(rubber) of the brakes lines could suffer internal
failure...I think that is a long shot. I may change my mind but I'm going with the power booster replacement for now.

Jim

[wxman1]

Oh...one thing...the front flexible lines(rubber) are new. They came with the
brake upgrade.

Jim

[volksworld]

oops...sorry. hadn't noticed it was a conversion and had new hoses...think you're on the right track ...there needs to be clearance between pushrod and back of master...see if you can do a depth comparison between old and new ones..the master should contact the booster with no resistance...you shouldn't have to push it on

[68gmsee]

Originally Posted by wxman1

Oh...one thing...the front flexible lines(rubber) are new. They came with the brake upgrade. Jim

Gotcha. Temp on the brakes is about normal since they're supposed to lightly drag.

Here's a CPP (Classic Performance Products) article on the booster to brake M.C. adjustments:
http://www.classicperform.com/TechBo...ot.htm#pushrod