Power Brakes will not work

SHAVER7254

MEMBER
1990 CHEVROLET SILVERADO
190,000 MILES

We have had this problem for over a month and can not figure out what is wrong. We have changed the brake booster 3 times, the proportioning valve once and the master cylinder twice. We have let the brakes gravity bleed, we have bled the brakes using a second person for assistance, and we have changed all the brake lines. When we have the truck motor off, we have a full brake pedal, but as soon as we start the truck, the brake pedal goes to the floor. When we pull the vacuum line on the power booster, we have a full brake pedal, just not power brakes. We are at our wits end.

5 Replies

RIVERMIKERAT

MECHANIC
6,110 POSTS

Disc or drum rear brakes? Have you adjusted the rears lately? Do you make it a habit of using the parking brake all the time?

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Tuesday, December 27th, 2011 AT 12:17 AM

SHAVER7254

MEMBER
2 POSTS

Drum Rear brakes, yes they have been adjusted and as for the parking brake, we have no idea as we just acquired the truck and have been working on it for about 2 months. The brakes were the last thing to do before we got it road worthy. It also sat for about a year before we bought it, if that helps at all.

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Tuesday, December 27th, 2011 AT 12:37 AM

RIVERMIKERAT

MECHANIC
6,110 POSTS

Pull the rear drums and check the wheel cylinders. See if they're leaking.

Did you bench bleed the master cylinder prior to installation? During the bleeding process, did you allow the master to run dry?

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Tuesday, December 27th, 2011 AT 12:56 AM

SHAVER7254

MEMBER
2 POSTS

We have checked the wheel cylinders and they are not leaking. We did bench bleed the master cylinder and allow the master to run dry during the bleeding process.
We have perfect brakes until we put the vacuum line on the power booster, then we have no brakes and the brake pedal goes completely to the floor. When we take the vacuum line back off the power booster, we have perfect brakes and three-quarters of a brake pedal. We can't keep taking the power boosters back to the parts stores, as they are telling us there is nothing wrong with the parts and after three of them, we are beginning to believe them also!

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Tuesday, December 27th, 2011 AT 2:00 AM

RIVERMIKERAT

MECHANIC
6,110 POSTS

Ok. By allowing the master to bleed dry, you have introduced air back into the system.

What I would do is get two clean plastic bottles and fill them about a third of the way with new fluid. Puncture a hole in the top of the bottles and insert a 5/16 or 3/8 vacuum hose into each bottle until the end is submerged in the brake fluid. Attach each hose to the bleeder screw on the rear brakes, crack the bleeders open and work the pedal a few times.

You should have a helper for this, so the helper can observe the bottles for air bubbles. When there are no more bubbles, hold the pedal down and tighten the bleeders. Then repeat the process on the front.

One thing, with the power booster attached, the pedal will naturally sink closer to the floor when stepped on.

I would also double check the adjustment on the rear brakes. While turning the rear wheels by hand, you should hear the drums moving across the shoes basically full time. Yanking down on the back on the wheel should get you right around two revolutions. And more, and the brakes aren't properly adjusted. This will cause the pedal to need more travel before brake engagement.