My '72 Chevelle Malibu had 4-wheel manual drums. I converted to power front discs. Front discs work fine but cannot get rear brakes to work. I have proper power booster/master cylinder, new ball joints and spindles, new wheel cylinders, and new pads. The proportioning valve is from a friend's '67 SS with front discs and it worked for him.
We're tearing out our hair because can't figure out the problem. Would appreciate suggestions/ideas. Did I miss something?
Thanks in advance
Bob


'72 Chevelle Malibu/355-4-bolt main/Edelbrock cam/Keith Black flattops/Trick Flow heads/Hedman headers/700R4 tranny/3.71 gears.

Are you getting fluid to the rear whl cyls? You may have a bunch of air trapped, try gravity bleeding the system. Get a glass jar for each rear wheel, clear plastic hose that will fit over the head of the bleeders tight. Place the other end into the glass jars, fill the jars with brake fluid till it covers the end of the hoses. Top mast. cyl. off, open bleeders, keep mast. cyl. full and see if any air comes out. Just sit back and wait it may take awhile to get all the trapped air out. If not, start at the mast. cyl. make sure it is pushing fluid out to the rear. It is possible for the front side of the mast. cyl. to be working and not the rear. Next go to the proportioning valve and make sure fluid is getting to the rear circuit of it. Make sure fluid is also coming out of the valve, going to your rear brks. If all is well, move to where the steel line connects to the rubber one over the rear axle and make sure fluid is coming out. Good luck. Thomas

On my car the Flexible line that goes from the Body to the rear-end got clogged when I did my swap.

Thanks for the response guys. We've bled the system numerous times (master cylinder and brake lines). Changed boosters, changed master cylinders, changed proportioning valves, and installed new brake lines. Got rid of all air bubbles. There is lots of fluid to rear brakes. The only other thing I can think of is to try another proportioning valve. Mine didn't do anything for the rear brakes and neither did my buddie's and it is brand new. That's why we're tearing out our hair:confused:.
Any other ideas/suggestions?
Thanks.....Bob

Are the drums, in specs, not too thin? Adjusters on the right sides? Have seen people mix them up and wind up backing them off when they adjusted them. Every time you back up, it would loosen the brakes. Thomas

plunger/Rod too short?

turbopowered68 and ls5-70SS: Thanks for your replies.
Rod is right length and drums are practically brand new. Brakes worked fine until we went to a disc/drum conversion. Now nothing but frustration. The brake pedal feels harder than it should and of course no rear brakes.
In a previous post a particular proportioning valve was recommended to a guy with a '72 who was about to convert from drums to disc/drums. I've ordered that valve but will tell you it looks exactly what we already have installed. This will be the 3rd proportioning valve I've tried. Have already been thru 3 boosters and 3 master cylinders. None of us can figure what else we can do to solve this problem.
Bob

You shouldn't be using a valve from a 67, the 71-72 used a combination valve mounted to the frame.

How about an adjustable prop valve?

I'm a believer in using original parts where possible. GM designed things a certain way for a reason...because they work. I've never had a problem using a stock setup for the brakes.

With all you have replaced, and the fact that the rear was working, as a drum set up. Getting fluid to the rear, no lines busted or blocked. About all it can be is a pressure issue. For some reason, your no getting no pressure to the rear. Either the wheel cylinders are not expanding (stuck or gummed up), but you have new cylinders. New master and booster. Is the master a dual compartment cylinder? One compartement for the disc and one for the drums. If so, it sounds like the only thing it could be is the proportioning valve. You might want to find one for a 71-72 Chevelle or Monte Carlo. Do not know what else it could be. Good luck! Thomas

Thanks for the additional replies from all of you. I've been away for awhile so here's an update. Front discs and rear drums are now working but not properly. The power disc brake setup is from a Chevy S-10 (spindles, ball joints, rotors, calipers) and I bought new calipers. Proportioning valve, master cylinder (2 compartment), and booster are correct for a '72 Chevelle. All parts, except spindles and ball joints are new. Vacuum is better than 14 hg and is taken from the carb. Brakes feel like they should feel when first applied but pedal gets hard and much more effort is required to stop the car when brakes are applied repeatedly such as in stop-go traffic. The only thing we haven't done is use a mastercylinder from an S-10 but don't know why that would make a difference.
Further suggestions would be appreciated from anyone. Thanks.
Bob

On my 71, i converted from power disc to manual disc, big cam, not enough vacuum. On the brake pedal there is 2 mounting holes for the m/c rod, 1 for power and 1 for manual.

Lower hole is for power brakes.

Do you have the check valve to the brake booster installed?