Hoping someone has some insight into this: I did a Disc Brake and Power Brake upgrade on my Wifes 61 Belair. 7" Booster, Corvette Master, Adjustable Proportioning Valve, New Disc Brakes up front and New Rear Drums on a 9" out back. So all done I now have power Disc/Drum setup Replaced some damaged and Old Brake Lines, New Brake Hoses to Caliper etc. etc. I have an adjustable Proportioning Valve for the rear drums and No residual valves. I bench bled the master, and then a buddy and I spent a tonne of time manually bleeding the brakes. I am very sure there is no air in the lines. I have a pretty soft pedal and it almost goes to the floor before it becomes firm and last night I took the car around the block hit the brakes firmly and the rear shoes stayed on, probably wasn't going more than 10-15mph. Just looking for input as I am at a loss! I did the same set up on my 56 Merc P/U with a swing pedal off the fire wall and it works like a charm. Thanks in advance for any advice
It might be a bad master cylinder just because its "rebuilt" doesn't mean its worth a damn. I would be inclined to say that its probably just air in the system. Did you replace all the flexible lines? How did you bleed the brakes? Just pump up the master and crack the lines? If you bleed the brakes properly the pedal should have gotten firm, all other things being in working order. How much time did you spend bleeding the brakes? I've spent several hours bleeding brakes when the system started dry to get good pedal feel. Where did you get the proportioning valve? and when you say the rear shoes "stayed on" do you mean the rear brakes locked up and wouldn't release? sorry for the m*** of questions I'm just trying to get a feel for what your dealing with.
residual valves are only necessary when the master cylinder is lower than the brake system as with a under floor mount. you can use them on a firewall mount, but its really not necessary.
Did you bench bleed the master cylinder before you installed it ? If not you still have air trapped in cylinder.
Actually you need the 10 lb. residual valve in the rear drum brake system to keep some pressure on from the wheel cylinders to the drums so they don't push all the way back into the bore and **** air.
Yep that has been pretty well standard on these setups for years. Unless the Corvette master cylinder is designed for a disk/drum setup there is nothing to retain the pressure needed to keep the rear brakes from drawing air in past the wheel cylinder cups. Also, are rear brakes properly adjusted? it seems to be pretty common on here that guys aren't properly adjusting the shoes on drum brakes before they attempt to bleed the brakes.
My fault the vette master cylinders only had the built in residual valve up to '63 or '64 and the one you probably have is for a disc/disc setup. so yea a 10 lbs residual valve would be a good idea. I still don't care for the blanket statement that drums need a residual valve there are plenty of master cylinders with it built in. and you should consider this when looking for a master cylinder instead of having to buy a external residual valve to get a disc/disc master to work with drums.
I have probably spent about 3 hours over the past couple of days bleeding the brakes and there is firm pedal at the bottom of the stroke, I did not think I needed residual valves as the Master is the highest point on the system. The Master is brand new not rebuild I just replaced it as the first one I bought was leaking from the rear seal. The rear shoes di dnot lock up they just started dragging, if I let off the gas it will coast to a stop in about 15 feet! I did adjust the the rear shoes (properly I think) they are just starting drag when the wheels are turning by hand. Think I migh try the residual valves but when I bought the MC it is for a disc/drum set up. What about the push rod adjustment, could this be the issue for the low pedal? Cheers guys for all the advice I will check this stuff when I get home tonight Thanks
merc56, I always dis***emble master cylinders, new or rebuilt, to make sure the bore and internal parts are squeaky clean and damage free. Generally new are OK, rebuilds are not. If fact I would stay away from rebuilds period! Are you sure the pedal bottoms the master pistons BEFORE it hits the floor or bracket? Gotta make sure. This is something I "harp" about, but is a common cause of excess pedal travel because of a poor bleed. As noted, I would look at your rears, 'cause sompin's not right back there! Make sure ***embly is correct, short shoe (lining) to the front, and good adjustment, about 10 "clicks" off from a full tight drum will give you about .025"-.030" shoe-to-drum. As usual, lots of bogus info on residuals. They (2 lb) should ONLY installed on disc systems when the master is below the floor and lower than the calipers, but 10 lb residuals should be installed on ANY drum system, (unless already internally installed in the master) and WERE (internally in the master), until metal cup expanders made them obsolete in the early-mid 70's. NO, they do NOT "hold the shoes close to the drums" (how can a 10-15 lb check valve overcome 75-100 lb or more return springs?) No, they do NOT keep the wheel cylinder pistons from "being pushed back too far in the bore". The mechanical design (shoes, anchor, adjuster) does that. What residuals do is keep air from being pulled past the cups during fast brake release, as has been noted above. They also help firm up the hydraulic system which results in slightly less initial pedal travel. One more thing to check is to make sure the booster push rod has '040"-'050" clearance to the primary master piston, and this should be checked with full vacuum (approx. 20 " Hg) applied to the booster, and pedal FULLY released.
It's the booster OUTPUT push rod to master cylinder piston clearance that is measured. We had a fixture with a dial indicator at work that fit over the booster's cylinder studs, in place of the master. But you can do the same thing with a 6" dial caliper or depth gauge and a piece of flat stock.
The master cylinder can't be for a Corvette if it is for a disc/drum setup. Corvettes never had a combination system, either all drum or all disc brakes. And all 67's and later were disc brake utilizing a dual master cylinder configuration. 1965 was the last year that drums were available on Corvettes. Starting in 1966 they were all disc brake car, but with a single master cylinder configuration that they shared for 1965 and 1966.
Guys thanks for all the help, got her sorted out last night. I never buy rebuilt MC's, the bad one I had was New same with the replacement. It was the booster push rod, it needed some more adjustment. Car stops on a dime now. Thanks again. Keith
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