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Old 02-13-2009, 05:56 PM
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Rear Disc Brake Info Needed

I have a 81 camaro just put disc brakes on rear the car had been sitting for quite some time and rear brake lines were shot. I replaced all lines to the rear and im not getting any brakes in rear. i am getting fluid to each caliper but it just trickles out when bleeding them. was told my brake booster could be bad but front brakes work and when cover is off and you press the brake it will shoot a stream 6 feet in the air. Also took apart the praportioning valve under brake boost and it looks to be clean and the valve in there works back and fourth. i have looked at all the parts stores around no one can get me a new praportioning valve and chevy dealership said they cannot even get one. do I need a differant brake booster because of putting disc on rear. Any info will help thanks.
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Old 02-14-2009, 01:48 AM
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Quote:
Originally Posted by Camaro6.6L
I have a 81 camaro just put disc brakes on rear the car had been sitting for quite some time and rear brake lines were shot. I replaced all lines to the rear and im not getting any brakes in rear. i am getting fluid to each caliper but it just trickles out when bleeding them. was told my brake booster could be bad but front brakes work and when cover is off and you press the brake it will shoot a stream 6 feet in the air. Also took apart the praportioning valve under brake boost and it looks to be clean and the valve in there works back and fourth. i have looked at all the parts stores around no one can get me a new praportioning valve and chevy dealership said they cannot even get one. do I need a differant brake booster because of putting disc on rear. Any info will help thanks.

I think it kind of sounds like a master cylinder problem.
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Old 02-14-2009, 03:11 AM
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I think you need a different master cylinder that is designed for rear disc brakes. I know the master cylinders for 4 wheel disc brakes on late 70's Trans Ams were different than the disc/drum master cylinder.
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Old 02-15-2009, 04:43 PM
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Disc Brakes

My buddy replaced his rear drum with disc and no problems. I also was told that I should remove my proportioning valve. I have a new master cylinder on order will try that first. When cover is off master cylinder should it shoot a stream of brake fluid 8 feet in the air when i press on the brake.
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Old 02-15-2009, 05:10 PM
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Yes ditch the stock combination valve, If it was for disc/drum it has a metering valve in it that will mess with your low speed braking.
If you are getting only a trickle to the rear either the shuttle valve is not centered, or the proportioning valve section is trash. Either way, the stock disc/drum combination valve needs to go away.
You can replace it with a disc/disc combination valve or an adjustable prop valve to the rear lines only or just plumb it straight..I'd want at least prop valve in it if it were me..

You can get the combination valves from master power brake, SSBC, or brake tech solutions. I think CCP sells them too. The adjustable valves are available from jegs, speedway, summit, most places that sell brake parts sell them.
You can probably use the stock MC, but if the car has been sitting for 2 years the master is probably shot. If it shoots a geyser of brake fluid out the resevoir rear seal on the front piston is probably trash.
Get a master cylinder for 4 wheel brakes, the only real difference is the rear outlet residual pressure valve is not in the disc/disc master.

The power booster is probably sized ok, the hydraulic system operating pressures won't be any higher with the addition of rear disc than they were with front disc only.

Later, mikey
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Old 02-15-2009, 05:24 PM
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Quote:
Originally Posted by powerrodsmike
If it shoots a geyser of brake fluid out the resevoir rear seal on the front piston is probably trash.


Later, mikey
I just went through 3 different MCs for my Yukon. Finally I read the directions and it said (put cap on reservoir while bench bleeding to prevent the brake fluid from squirting out. )
Why would brake fluid squirt out of the reservoir? It should be going the other way. It makes no sense to me but it does. If the fluid is low in the reservoir then it will squirt 6' in the air.
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Old 02-15-2009, 05:50 PM
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All the others have said. Then you may want to check out your calipers. They could be partially plugged by rust from the old lines or rusted severely internally from sitting so long. Try removing your bleeder port and make sure it is clear.

Trees
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Old 02-15-2009, 06:42 PM
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Quote:
Originally Posted by weirdbeard
I just went through 3 different MCs for my Yukon. Finally I read the directions and it said (put cap on reservoir while bench bleeding to prevent the brake fluid from squirting out. )
Why would brake fluid squirt out of the reservoir? It should be going the other way. It makes no sense to me but it does. If the fluid is low in the reservoir then it will squirt 6' in the air.
Well it will squirt a little, until the transfer port is closed by the piston moving past it. but if it shoots alot, it could be bypassing internally. I have found that when I bench bleed a mc, if I stroke the pushrod too fast it will aerate the fluid, and shoot a fair height, so I do it slowly enough that it does not shoot out.

Normally the fluid being pushed by the rear piston is what pushes the front piston, that's how the pressure stays equalized in both sides of the system.

I'm not ruling out anything on the OP's car.

Later, mikey
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