The Stovebolt Site Forums The Parking Lot '60-'66 Trucks Master Cylinder?

I finally got the trans working right and the leak stopped on the C-10. Now we've got brake problems, the day we tried out the repaired trans, the front brakes started tightening up. They got tight enough that it was hard to pull itself. We stopped for a few minutes and they loosened up enough to drive on home. Now the pedal goes to the floor and only the rears work. There is very little fluid at the caliper bleeder when the pedal is pushed. I tried to reverse bleed with a big syringe and couldn't get fluid to go to the MC. I took the line loose at the MC and easily pushed fluid back up to it. I'm thinking something is blocked in the master restricting the flow. Does that sound logical?

if it's been sitting a while the lines might have sludge/ contamination in them. if it's a loooong time the fluid turns to a white chalk. when i start driving a "field special" i take each line loose and clean the lines with alcohol and compressed air. then rebuild each wheel cly.
i noticed you mentioned caliper have you changed the front brakes over to disc brakes? if soit might have something to do with that, if it's been recently changed. good luck

It's all new stuff. The front end is out of a 72 C-10 with a CPP brake kit with 6 lug discs. It has a 8" booster with Corvette master cyl. and all new lines. I have a brake control valve in the rear line.

Your pushrod between the MC & the booster is probably too long holding the master cyl on a little.

I think it was when they locked up. I shortened it 1/2" this morning and got the pedal free play about right. Now I just can't get front brakes.

Hy Mearl, did you install a combination valve with your new master cylinder? It sounds like the valve may have shuttled over shutting off flow to the front brakes. If you can generate any pressure in the front brakes, you could slowly open a rear brake bleed screw, this would mean less pressure in the rear system than the front system, and the valve should shuttle towards the center if not all the way to the rear, you may have to chase it back and forth before you get it in the center. Once you think it is in the center close all the bleeders tightly and slowly step on the brake pedal to generate pressure, this should finally center the valve, hope that helps

I didn't use a combination valve, just the adjustable valve to the rear wheels. I thought that would be enough. Now when the pedal is pushed down, I can see the calipers move just a tiny bit at the very bottom of the pedal travel.

Hy Mearl, I think you should remove the lines from the master cylinder and bleed the master cylinder alone. Do you have the primary system [closest to the firewall] activating the front brakes?

I have the front chamber going to the front brakes. That's the way I thought they were supposed to be. I was going to bleed the master with the lines off but I couldn't find my bench bleed fittings.

Hy Mearl, the primary system [closest to the fire wall] should be plumbed to the front brakes. As you are using a master cylinder designed for a vehicle with disc brakes all around, I would suggest a 10 p.s.i. residual pressure valve be installed in the rear brake system, as a minimum addition to what you have already done. I highly recommend a combination valve for a disc drum system be installed, this would give you the metering function your adjustable proportioning valve is now doing, and if you hooked up the warning light circuit, it would provide you with a visual signal of system problems. Hope that helps.

I looked at a couple of my old cars, Trans Ams & a 69 El Camino, they all have the larger chamber which is the front, going to the front brakes. Of course they also have proportioning valves. I thought since both chambers are the same size on the Vette cylinder, it wouldn't make any difference. That's why I put the adjustable valve on the rear. Another thing I may have screwed up is putting big lines on the front and small on the rear, would that be backwards? I was thinking that the calipers needed more fluid. If you still think I need to swap the chambers, I'll try that this weekend.

Hy Mearl, because the master cylinder you're using was designed for a disc/disc application, both chambers would be close to the same volume, if not identical. Wherever possible I try to copy factory engineering, the only exception to that would be to make something bigger/stronger, so I would use the same size lines as the factory used. I still think you should bleed the master cylinder alone, before you attempt anything else. Once we went to disc/drum vehicles, you are correct the larger volume cylinder was used for the front brakes, on the drum/drum master cylinders the primary system activates the front brakes. I think you may have some air trapped in the master cylinder. I still recommend the installation of a combination valve, but that is ultimately your choice, good luck, and let us know how it goes.

I took the master cylinder off and bled it on the vise. The rear worked as it should and pushed a strong stream of fluid when I held it out of the chamber while pushing. The front didn't do any thing, no bubbles or fluid.

Hy Mearl, it sounds like you have a failed seal in the secondary side of your master cylinder. I would send it back as a failed new part, and get them to send you another one.

I'm glad to finally get it narrowed down, finding the time during the week is the hard part. I can't send it back, I've had it a couple of years and it did work. I don't mind buying another one. 3B thanks for all your help on this one, it would be nice if this thread helps someone else.

Good news on the 65 C-10 brakes. We replaced the defective Vette master cylinder with a reman one from the auto parts. This one is for a 71-early 80's chevy pickup. The primary chamber is the rear one and is larger than the front. I switched the lines around to fit it and put it on after bench bleeding. We had a good pedal before we even bled the brakes! Now they are exactly as they should be.
I think I learned some lessons here. I don't think a Corvette cylinder is the best idea for disc/drum because there is no primary chamber due to the 4 wheel discs.
Anyway Deb got to drive her truck again and now I can get to one of my projects. I just wish she'd finish the bodywork on the tailgate.

Hy Mearl, that is great news, which I'm glad to see/hear, continued good luck.

sounds like no freeplay in the pedal linkage to me, pedal should move at least 1/8 inch before it does anything..