|10-20-2011 08:14 AM|
The play from the end of the pushrod to the bottom of the hole in the master cylinder piston should be set at the minimum amount required to insure that the piston fully returns when the brake pedal has been released.
Depending on the pedal ratio, 1/8" clearance at the end of the pushrod could easily be over an inch of travel at the pedal BEFORE the master cylinder piston even begins to move. This could result in running out of pedal travel before the piston in the master cylinder has traveled far enough to fully apply the brakes.
The master cylinder piston diameter and travel also need to be of sufficient size to allow the wheel cylinders/calipers to be able to move enough to enable the shoes/pads to have good contact with the drums/rotors and have sufficient applied pressure to stop the rotational forces. i.e. Master cylinders designed for drum brakes often do not have sufficient capacity to fully operate disc brake calipers.
|10-18-2011 02:36 PM|
learned a little trivia, for Studebakers anyway. The pushrod has to be adjusted to near zero play. I guess it's a combination of pedal length, pushrod length, and MC size, any play and it just doesn't engage the brakes. 1/4 inch of adjustment the the difference was amazing.
|10-16-2011 03:33 PM|
|10-16-2011 11:15 AM|
Thanks for the help guys. IN response, Frisco, every thing is as you suggest.
No residual in the MC, blue 2# to the front. The brakes now grab and release as they are supposed to (once I changed the MC) About 1/8 inch of freeplay before the rod engages the MC. Pull it down by hand, from below and the pads engage quite quickly and hold strong enough I cant turn the rotor by hand, but inside the car, I can push the pedal all the way to the floor. Some resistance, but never a hard pedal. It's like the system has "give" in it somewhere. But it is a solid frame mounted MC. Yesterday I installed new calipers and pads. No change what so ever. [ Every line, hose, fitting is new. There are absolutely no leaks, and the system doesn't"pump up" like it has air in it. ]
T bucket, it's possible the new MC is bad, but early on (3 days ago) I plugged the ports. When I plugged the front, with the rears hooked up, the pedal was high and rock hard (telling me the front port was good?) When I plugged the back port with the fronts hooked up, the pedal went to the floor, telling me it was in the front system?
OH, and yes, I got a good stream of fluid when my wife pumped the brakes. But I also borrowed a power bleeder that pushes an unlimited stream thru the system.
This isn't my first rodeo, Ive even done the anal, like bleeding the junction block, bleeding the brake light switch, tilting the rotors back so the bleeders are nearly straight up. I'm positive there is no air in the system.
Even tho I bench bled the MC, I suspected it. So I start every bleed at the MC. It's my understanding a power bleeder will bleed the MC also?
|10-15-2011 06:58 PM|
|T-bucket23||Dont assume the new master is good. If you attempt to bleed the brakes do you get fluid out of the bleeders|
|10-15-2011 11:43 AM|
If the pushrod attached to the brake pedal to the master cylinder is too long, it will 'hold' the brakes on slightly even with your foot off the brake pedal. This will not allow the calipers to fully return and will cause the pads to drag. They could actually lock up. Excessive heat and possible warpage of the rotors can occur.
The next thing to check is the residual valves. If you have 10 p.s.i. valves on the front lines to the calipers you will also cause the brakes to drag. In most cases where the master cylinder is installed on the firewall and above the calipers there is no need for a residual valve to be in the system. If the master cylinder is installed below the floor (as you write), then for disc brake applications a 2 p.s.i. residual valve is needed. Be sure that the master cylinder doesn't already have a residual valve installed in it. If it does it will be a 10 p.s.i. unit and can be seen just inside the outlet connection. It should be removed. A 10 p.s.i. residual valve is for drum brakes. It is also needed when GM rear calipers (the ones with the built in parking brake) are installed.
If you have disc front and drum rear brakes, then you should have a 2 p.s.i. residual valve in the front lines and a 10 p.s.i. residual valve in the rear lines. The residual valves should be installed close to the master cylinder.
You mention a proportioning valve. If it is an aftermarket adjustable one, it should be installed in the rear brake line only, after the residual valve and also close to the master cylinder.
On the dual reservoir master cylinder; if one of the reservoirs is larger than the other, the larger reservoir is for the front disc brake calipers. If the reservoirs are both the same size, it more than likely is for a 4 wheel disc brake setup.
As to the calipers working properly. Check the clearance between the pads and the rotor with no brakes applied. It should be between .015 and .030. Apply the brakes and then release. Re-check the clearances. If the clearances are now closer, then the calipers are not retracting properly.
|10-15-2011 11:28 AM|
My buddy bought this 61 Studebaker hawk a month ago. Brakes were fine.
Front disc brake conversion, frame mounted dual reservoir master cylinder, residual valves and proportioning valve. It developed a brake problem, calipers wouldn't release. Definitley a bad master cylinder. Replaced it with an identical new, not rebuilt, unit. Couldn't get the brakes to bleed out properly. Found some single flare ends in the system and replaced every fitting and every brake line,basically built a new hydraulic system. Still won't hold pressure.
The only thing left are the calipers. But they show no signs of leaks and appear to be working properly. [He found the original problem when he touched a front wheel and nearly burned his hand, brakes were locked up] . Maybe the heat warped the seals?
Is it possible for the seals in the calipers to not hold pressure, and still not leak? Any other possibilities?