I just finished my vacume convertion and tried to gravity bleed them and went for a test drive. The rears were sticky like before but I could hold 5-7 lbs of boost so I clamped the proportioning valve and bled the brakes by slowly pumping and holding and I adjusted the shoes up a little tighter to the drum. I did the rears only. I took the clamp off and went for a drive and no more sticky rears but I cant even hold any boost. What has changed to cause this? Brakes feel good for driving but not to hold boost. I have s 10 cyl. and long shoes. Also have a line lock but that worked fine. I T eed into the PCV for vacume and the check valve is working. I used a Autozone reman, Any ideas? TIA
Holding boost w vacume brakes
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stickybones
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- Dec 27, 2001
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Why did you only bleed the rears?
stickybones
What?
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- Dec 27, 2001
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- 1,069
Not sure what you mean by "sticky rears".
Bleeding the brakes is a relatively simple job that alot of people mess up. Try doing it again. Do it the old fashioned way, as you've already done and push the brake pedal down slowly while someone else cracks the bleeder (or you crack the bleeder while someone else pushes the brake pedal). Don't "pump" the brakes. I have speed bleeders on the rear and that makes the rears a one man job at least.
I heard you clamp the proportioning valve plunger when pressure bleeding the fronts, not the rears.
Did you use "used" parts? Are you sure they're good? If the brakes don't feel hard and they work, I'd assume the booster is fine. The master cylinder may not be though.
Are the rubber hoses to the front calipers and the one central rubber hose in rear in good shape?
I assume there are no external leaks anywhere.
As you've done, it's a good idea to expand the rear shoes as far out as possible so the drum barely fits over them. For my car, I needed the drums cut to true them up to get the final stopping power I needed (thanks TurboJim). What I'm saying is that the condition of your drums is something else to consider.
The way I see it, the fronts "could" be bad and that may affect how the rears hold...and vice versa. A defect anywhere in the system could affect the whole system.
Good luck. I went through a few headaches when I converted to vacuum brakes too. I bleed the brakes 3 or 4 times, changed the MC twice, replaced the wheel cylinders, adjusted the rear shoes and cut the drums; all to get good braking. I can now hold ~8psi with street tires and can lock them up in a panic stop (not necessarily a good thing, but at least I know they work to their maximum potential). Haven't been to the track since the conversion so I don't know how well my QTPs will hold.
Bleeding the brakes is a relatively simple job that alot of people mess up. Try doing it again. Do it the old fashioned way, as you've already done and push the brake pedal down slowly while someone else cracks the bleeder (or you crack the bleeder while someone else pushes the brake pedal). Don't "pump" the brakes. I have speed bleeders on the rear and that makes the rears a one man job at least.
I heard you clamp the proportioning valve plunger when pressure bleeding the fronts, not the rears.
Did you use "used" parts? Are you sure they're good? If the brakes don't feel hard and they work, I'd assume the booster is fine. The master cylinder may not be though.
Are the rubber hoses to the front calipers and the one central rubber hose in rear in good shape?
I assume there are no external leaks anywhere.
As you've done, it's a good idea to expand the rear shoes as far out as possible so the drum barely fits over them. For my car, I needed the drums cut to true them up to get the final stopping power I needed (thanks TurboJim). What I'm saying is that the condition of your drums is something else to consider.
The way I see it, the fronts "could" be bad and that may affect how the rears hold...and vice versa. A defect anywhere in the system could affect the whole system.
Good luck. I went through a few headaches when I converted to vacuum brakes too. I bleed the brakes 3 or 4 times, changed the MC twice, replaced the wheel cylinders, adjusted the rear shoes and cut the drums; all to get good braking. I can now hold ~8psi with street tires and can lock them up in a panic stop (not necessarily a good thing, but at least I know they work to their maximum potential). Haven't been to the track since the conversion so I don't know how well my QTPs will hold.
GNVYUS 1
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- Dec 15, 2002
- Messages
- 4,325
You went from the Power Master that has around 2000psi of pressure to vacuum which is less than half that pressure, thats why you can not hold as much boost.
Make sure you have both shoes in the drum the BIG shoes.
Make sure they are adjusted and bleed properly.
The secret to holding boost with vacuum brakes is getting peak vacuum and then hammering the brakes.
SO< Put the car in neutral, hit the gas, when the vacuum peaks hammer the brake, then put in gear and boost. If you do not have a boost gauge that reads vacuum, hammer the brake right after peak rpms were reached.
Make sure you have both shoes in the drum the BIG shoes.
Make sure they are adjusted and bleed properly.
The secret to holding boost with vacuum brakes is getting peak vacuum and then hammering the brakes.
SO< Put the car in neutral, hit the gas, when the vacuum peaks hammer the brake, then put in gear and boost. If you do not have a boost gauge that reads vacuum, hammer the brake right after peak rpms were reached.
All parts are new. Everything seems to be in good condition. I bled the rears with the prop valve clamped, I figured I would go throu and bleed them all again as far as for sticky brakes that means when I pull out of the garage and barely hi them the rears lock up but after I drive a few miles they are just touchy. So you need the clamp the valve to bleed the front not the rear? Last time I bled them I didnt know about the valve and it took forever to get the rears to bleed.
