Rear brakes won't bleed

[Whistler]

I have Aerospace 4 piston fronts and a 01 Z28 master cylinder already installed on my car, and still have the stock drums on the rear. I have a complete 01 Z28 rear brake package ready to go, but am getting ready for a dyno challenge this weekend, so I'm trying to get by with the drums temporarily.

Problem is I am getting very little pedal when bleeding, and the front calipers are only just starting to grab as the pedal hits the floor. The front brakes bleed fine, but the rears always seem to have a little bit of air in them, no matter how much I bleed them. I'm thinking the rear cylinders could be shot and letting air in when I try and bleed, but not sure. I guess I'm going to have to add a rear disc conversion to the list of things to do this week..... any suggestions?

[paul67]

Is the master cyl disc front disc rear set up if so you need a vavle fitted for the drums that goes in side the master.
paul67

[sporter]

You need a 10 lb residual valve in the rear brake circuit if you have drum brakes. Without the 10 psi in the drum circuit, the wheel cylinders will suck air everytime you let off the brakes quickly (like when you are bleeding them). You can get the valves by themselves or shop for a master cylinder that has one in it for rear drum brakes.

The rear drum brakes found on modern cars that still use drums do not need the residual valve because they have expanders on the seals in the wheel cylinders. Not sure when they started using expanders but the wheel cylinders in my 1967 Firebird don't have them but the wheel cylinders in the 1967 big Pontiacs did have them.

[Whistler]

Thanks for the assistance. I got the car on a lift and the wheel cylinders were shot. I don't know if they were already done, or if the lack of that valve did them in... either way I wasn't about to spend any time fixing drums, so I did the LS1 discs.

Now my rally wheels don't fit. I can't afford the rims I want for a few more months, so I've got to come up with something temporary to get it off the lift. :(

But al least now I've got a nice firm brake peadl... something this car hasn't had in years