1989 Chevrolet Chevelle brake pedal goes to floor

Tiny
FUZZY57
  • MEMBER
  • 1989 CHEVROLET CHEVELLE
  • 70,000 MILES
Not losing any fluid, bled brakes several times replaced booster and brake master cylinder, bench bled master cylinder I think I got air somewhere can't get it out could the hoses be causing a problem, I am kind of stumped where to go from here, would an obstruction in lines cause this?
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Wednesday, September 18th, 2013 AT 8:23 AM

7 Replies

Tiny
CARADIODOC
  • EXPERT
Was this a used or a rebuilt master cylinder?
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Wednesday, September 18th, 2013 AT 12:19 PM
Tiny
FUZZY57
  • MEMBER
Rebuilt master cylinder
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Wednesday, September 18th, 2013 AT 3:35 PM
Tiny
FUZZY57
  • MEMBER
Started to bleed brakes again left rear, start to turn the bleeder screw then a burst of pressure comes out blows the line off of the bleeder screw sometimes
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Wednesday, September 18th, 2013 AT 3:39 PM
Tiny
CARADIODOC
  • EXPERT
What model exactly are we discussing? There was no Chevelle in 1989.
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Wednesday, September 18th, 2013 AT 11:20 PM
Tiny
FUZZY57
  • MEMBER
I am sorry didn't proof read main question it is supposed to be cavalier, how do I change main question?
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Thursday, September 19th, 2013 AT 5:22 AM
Tiny
CARADIODOC
  • EXPERT
This is fine. You and I are the only ones who can see this conversation. If you don't like my replies you can repost a new question so everyone can see it and have a chance to respond.

GM has an unusual master cylinder design on their front-wheel-drive cars. I think this includes an '89 model. There is a valve in it that will move to block off two ports when there is unequal pressure in the two hydraulic systems. That is a nice feature if you spring a leak because it will prevent brake fluid from being lost, but if you bleed the system improperly it will also trip that valve. Most front-wheel-drive cars use a "split-diagonal" hydraulic system meaning the left front and the right rear brake are on the same system, and the right front and left rear are on the other. They aren't split front and rear like on older cars and trucks. Due to design changes in "scrub radius", a non-adjustable alignment angle, you will not have a brake pull when one front brake isn't working.

Pedal-bleeding on the car, with a helper, and bench-bleeding the master cylinder will trip that valve. If that happened during bench-bleeding, there could be air still in that part of the master cylinder and there's no way to get it out until that valve is reset. The only way I've found that always works is to go to one of the wheels that isn't flowing any brake fluid when you open the bleeder screw, and give it a very short, quick burst of compressed air, then let it gravity-bleed.

With any master cylinder you should never run the brake pedal more than half way to the floor to avoid damaging the lip seals on the crud and corrosion that build up in the bores where the pistons don't normally travel. With the GM master cylinders you can also avoid tripping that valve by not pushing the pedal more than half way.

The first thing is to verify brake fluid is flowing from both front calipers. I suspect you'll find no fluid flows from one of them and you'll have to reset the valve with compressed air. Once you reset that valve, crack open the cover on the master cylinder reservoir so the fluid can run down freely, and open both front bleeder screws, then wait for the air bubbles to stop showing up. At that point, close the bleeder screws, push the brake pedal about one quarter way down very slowly, wait a few seconds, then let it release quickly. That will push brake fluid down, air bubbles will float back up, and when the pedal is released any air bubbles will wash back up into the reservoir. Do that multiple times until the pedal starts to feel more solid. Now you can continue to bleed at the wheels if it is still necessary.
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Thursday, September 19th, 2013 AT 1:20 PM
Tiny
FUZZY57
  • MEMBER
Thank you for your help had fluid at both calipers kept bleeding the rear brake cylinders open the bleeding screw turned it about a 1/8 of turn kept getting air it was sucking air around the threads or so I think pedal was still going to almost the floor took a test drive it was going down about halfway even with the accelerator pedal good enough for now thank you again!
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Sunday, September 22nd, 2013 AT 5:21 PM

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