[71corvette]

I have completely replace the brake system on my 36 sedan. new aftermarket power booster with dual corvette style master cylinder and new GM porportioning valve and new lines.
I have a nova rear end with all new drum brakes and wheel cylinders and disc in front.
now the problem, I cant seem to get a good pedal and you have to pump the brakes once to stop. I have bled 2 quarts of fluid thru the system, checked the rear brake adjustment and tried another master cyl. the strange thing that happens is when you bleed the rear you loose all your pedal for a while then it builds back up. I am not getting air just fluid.

[powerrodsmike]

Is the pedal spongy, or hard and just not actuating the brakes? Are the rear wheel cylinders perfectly horizontal? If they are not you won't get all the air out through the bleeders.

I've had a couple of cars that I actually had dismount the wheel cylinders to bleed them, the axle was that far rotated. Sometimes when someone shortens a rear end the housing ends get welded on in a position that puts the backing plates on weird..

Do you have an adjustable proportioning valve in the rear line? Sometimes those go bad too, but usually when they do you can't bleed the rears.

Also, check for frozen anchor pins, or a caliper that is not free in the bracket. On the fronts, when the attachment points get sticky the pedal gets longer, because it has to take up the slack in the bracketry before it actually gets the pads on the rotors.

Just a couple things I'd check first.

later, mikey

[T-bucket23]

You may also want to be sure the front calipers are on the proper sides. If not the bleeder will not be positioned properly and you will never get the air out.

[71corvette]

the brakes are not spongy but there is just lots of pedal travel before you get brakes, they do stop and hold but its just a scary feeling. the wheel cylinders are level, brake calipers on the correct side and a regular porportion valve, non adjustable.

[powerrodsmike]

What size is the master cylinder? You really should have an 1 1/8" or 1 3/16" bore master cyl, especially if you are running one of the under floor master cylinder setups..From your other post, it looks like you have the early GM calipers, those have a big piston that takes up alot of fluid. IIRC the piston in those is 2 7/8" , and a GM metric caliper is smaller, at 2 5/16"..those are marginal with most under floor masters.

I see so many cars with under the floor 6 or 7:1 pedal ratios, which result in a relatively short master cylinder stroke..Then they put a 1" master, power brakes and the symptoms are pretty much what you describe.

I've fixed a few by making the pedal ratio closer to 4 or 5:1, and or putting a bigger master cylinder.


Later, mikey

[71corvette]

the master cyl/ booster was bought as a frame mount but I had clearance problems so ended up using a universal firewall mount bracket, I will have to check on the master size but it is one for power brakes

[cobalt327]

You might want to check to see if the front calipers are the "low drag" type that are supposed to use a stepped bore (aka "fast take-up") master cylinder, more: Test for low drag calipers

Also, concerning the rear brakes, there should be an internal residual pressure valve built into the port that goes to the drum brakes. If this is missing or not working right, the rear brakes will take up a lot of pedal travel (much like the fronts as described above) before the shoes actually make contact w/the drums.

In that case, a residual pressure valve can be used to maintain some line pressure that will prevent the slaves from fully retracting.

A last thing to check on is to be sure the rear brakes are correctly adjusted. There should be just the slightest whisper of shoe contact as the wheel is rotated.

[65ELCMO]

try a 10 lb residual valve in the rear line.

[71corvette]

ok the master is a one inch bore, where can you get a larger bore master that fits the universal booster? also I am not sure I understand pedal ratio and its affect other than a higher ratio would generate more force. I thought residual valves were only needed when the master cylinder was lower than the wheel cylinders.

[Cape Cod Bob]

I have the same problem . I have bled different ways and can't keep a pedal up. Now I just live with the double pump !

[barflymark]

All the drum brakes had a residual check valve. If you happened to get a 4 wheel disc brake master cyl. that is probably you problem. The previous posts that pointed this out were probably dead right.

[71corvette]

Originally Posted by barflymark

All the drum brakes had a residual check valve. If you happened to get a 4 wheel disc brake master cyl. that is probably you problem. The previous posts that pointed this out were probably dead right.

I dont think the master cylinder determines disc or drum type that should be determined by the proportioning valve.

[65ELCMO]

Some master cylinders had a check valve built in. The later one rely on the prop/combo valve. There are differant valves for disc/drum and disc/disc.
Something in your combination of parts is mismatched.

[powerrodsmike]

Most combination valves do not have residual pressure valves. Many, but not all master cylinders intended for use with drum brakes have an RPV in the port that leads to the drums...

The main difference between a disc/disc master cyl and a disc/drum master is the addition of an RPV in the rear outlet..

The main difference between a disc/disc combo valve and one for disc/drum is the addition of a metering valve, (or "stay-off" valve), in the front of a disc drum valve.

It's easy to tell if you have an RPV in the master, just take a paper clip or piece of wire and GENTLY probe into the conical brass seat of the MC outlet..if you feel a slightly springy resistance about 3/8" in, you have an RPV valve installed..if you can go into the outlet 1/2" or more, with no resistance , then there is none.

It is a good suggestion to add a 10# RPV in the rear line, but don't cheap out when you buy one..A while back I did some testing, and found that most of the RPVs you get leak down and seldom hold thier rated pressure. No matter which ones you get, the first check is to hold the valve up to a strong light, if you can see even the faintest sliver of light through the valve, it's no good. The second test is to make up a set of fittings with a 0-10# gauge to go in the output end of the valve,(a vacuum/fuel pressure gauge works well), and you can use your master cylinder as a pressure generating device to push some brake fluid through the valve, (don't blow up your gauge), then watch to see how much pressure stays on the output end of the valve...

I've tested a bunch like that, and find that if they do hold even 5 psi pressure, and don't leak down after about 10 minutes, then they are keepers.

You can also put a pressure gauge on the bleeder port, (you either have to buy or make an adapter that will seat in the bleeder port, but not allow pressure to leak past the seat or threads), and watch it there, but I find it's easier to bench test them first.

The chances that those Delco-Moraine calipers are low drag are slim...The only ones I ever heard of are metric GM.

The whole purpose of an RPV in a drum system ,(as originally designed by the OEMs who designed them), is to keep a little pressure on the cup seals, so that there is no chance air can leak past the seal on retraction...They will however take up a little slack in the pedal...not alot, but some.

A 10# valve in the front line of a car with low drag calipers will help a little too...Before you decide to put a 10# valve in a front line, you better make sure you have the low drag calipers..Cobalt already put up the link to a thread that tells you all about those things.

I'm still wondering about the MC bore size in the OPs system, that's where I'm putting my bet, as to what the real problem is..


Later, mikey

[71corvette]

Originally Posted by 65ELCMO

Some master cylinders had a check valve built in. The later one rely on the prop/combo valve. There are differant valves for disc/drum and disc/disc.
Something in your combination of parts is mismatched.

the porportioning valve was bought for a disc/drum system and is new. now who knows what they may have sent