Hi all. My buddy just had a 1970 all original LT1 dropped off at his house. It is his first vintage car and a survivor. He wants to keep it as original as possible to get a survivor rating by NCRS or whatever corvette organization does that. That might be a long shot as I am sure somethings have been changed on the car.

To the point, the brakes are very mushy. I am going to help him bleed the system and the person that dropped off the car mentioned that the proportioning valve might be locked up. He mentioned to break a line loose that goes into the valve and stomp on the brakes to center the piston inside the valve. What do you guys think? I was just going to bleed the master cylinder an then the calipers.

I don't think that is how the proportioning valve works. I think it's built with the correct proportioning and that's all you get. The moving parts are to detect a difference in pressure between the front and rear lines to trip the dash light.

I'd open the bleeders and make sure you have pressure to all 4 corners. Check the rubber hoses, old ones can cause mushiness. I don't know if the car has been on the road or if it's been sitting but give the brake system a thorough check and see how it comes out.

Tim.

It has been sitting. I will bleed all for corners. From my understanding the power brake vettes had two bleeders on each caliper and you must bleed the inside valve first. I will play with it. If he needs to rebuild the master cylinder that shouldn't be that hard.

Two bleeders? I'm totally out of my element here. :)

I guess take a careful look at what you have and what condition it is in and go from there.

Tim.

the component in question is not a proportioning valve... at lest not in name... on factory cars this was a accomplished by a combination valve and its name says it all. the combi valve does three things. 1: distributed fluid and pressure to the 3 circuits. Front left, front right, and rear. 2: acts as a proportioning valve that is preset by the factory, nonadjustable, and should be no need for, if getting a new one be sure it is the proper one for that specific year and model, as each vehicles could have a different proportioned pressure ratio. 3: a safety valve, in between the front and rear circuits is a metal piston at if either of the front or the rear circuits leak, loses pressure, that piston will move, close that leaking circuit to prevent you from draining your master cylinder.

As for fixing it the old man is right. Since it seems like the rear is not getting pressure, disconnect the front and GENTLY push the pedal. this should re center the piston. Reconnect the front and go find the leak in your rear brake plumbing.

It should also be noted that there is usually a sensor on the combi valves that would turn on the brake warning light, if its functional and connected of course.

More than likely has a caliper problem. Those older 4 piston calipers were very trouble prone. Seals were on the pistons, moving with them on the bore, rather than fixed in the housing with the piston moving on the seals. If it is a survivor, and hasn't had the calipers replaced with stainless sleeved units, you probably will have to. Jim.