Brake Pedal sinking to the floor occasionally, failing MC? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 12-12-2007, 11:35 AM   #1
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Default Brake Pedal sinking to the floor occasionally, failing MC?

Well, after seeing Wren'* post about his master cylinder faiing, I worried about mine. Well, I shouldn't have, because I think it'* failing. Today I was at a light with my foot on the brake and FWOOP! it started sinking quickly almost all the way to the floor. I lifted off and put my foot back on and it stayed in it'* normal position. From what it seems, it is very erratic, and only does it when it wants to. My Dad is saying my master cylinder is failing, as he took the Bonne the other day, and noted that he could feel the pedal sinking slowly. Any help is appreciated. Thanks!
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Old 12-12-2007, 11:38 AM   #2
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Sounds like mine when it failed.
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Old 12-12-2007, 11:43 AM   #3
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If you can, try bleeding first. If that doesn't help, go after the master. Look for brake fluid lieaks underneath where the master bolts to the booster. It may be very small leaks, but could still be there.

One of mine was obvious there when it was removed, the other wasn't. You can also unbolt the master and move it away from the booster to inspect it. The brake lines have enough flex in them. Don't move too far that you stress the fitting that re-fills the PMV reservior though.
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Old 12-12-2007, 11:56 AM   #4
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No wetness present, fluid is at normal level, no loss. I will probably pull the booster back a little bit later on and check it out. Thanks for the help.
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Old 12-12-2007, 12:00 PM   #5
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They can leak internally. A reman unit from NAPA (will be in stock) is going to run you about 40 bucks or so, plus core.

Make sure you know if it'* 6.5 or 8 inches before you go. You MUST bench bleed it before you install it.
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Old 12-12-2007, 12:07 PM   #6
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Yeah it'* the 8".

Found one at NAPA, $36.99 8" w/ABS.

Now for a big question, I have never bench bled a master cylinder, so I'll need to know how. I know everything but brakes and transmissions, those are my weaknesses.
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Old 12-12-2007, 12:13 PM   #7
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The kit will come with two new pins to hold the reservior to the MC, as well as two green fittings to screw into the MC, and two clear hoses. The instructions are mostly good.

Put the MC in a vise (upright) with the vice jaws on the bottom of the mounting flange, with the flange facing you so you can push on the piston (simulate the brake pedal). Install the green fittings and tubes. Fill the res (you must plug the outlet for the PMV reservior fill). Go about half full on the MC, but you may have to tip/tilt it to get the fluid to the front chamber also.

Put it back in the vice with the piston/flange facing you, and run both hoses into a small container of brake fluid, with the ends being under the surface (fully immersed).

Push the piston all the way in, hold for a couple seconds, and release. Wait for the bubbles in your little container to dissipate so you don't suck them back in on the next cycle. Repeat this process several times until there'* no more air. At that point, you can lift the tubes out, but leave them on. They'll keep it from leaking (make sure you hold them up so they're higher than the reservior as you install the MC). Bolt the MC on to the booster, then pull your plug underneath quickly, and hook up the 3/4" feeder to the PMV reservior, then remove the green fitting and tube closest to the firewall, and re-connect that brake line (rear first, it'* easier). Then do the same for the forward fitting.

Bleed the entire system.
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Old 12-12-2007, 05:25 PM   #8
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Quote:
Originally Posted by Bonneville92V688
Yeah it'* the 8".

Found one at NAPA, $36.99 8" w/ABS.

Now for a big question, I have never bench bled a master cylinder, so I'll need to know how. I know everything but brakes and transmissions, those are my weaknesses.
Do you have a part number? The best price I've found online was $70 with an outrageous core charge.

I also have the problem where hard braking will result occasionally in pedal to the floor... quickly... but no external leakage and my level is good. I suspect an internal problem in the MC. I'm looking at replacing mine in a couple weeks.

BKNJ
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Old 12-12-2007, 10:27 PM   #9
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Look at my general chat topic. You can see one of the part numbers in a pic or two. I don't have the boxes now though, they went back with the cores.

I got both rebuilt MC'* (one 6.5, one 8") for 145 including core. 100 for the pair after the cores went back. In stock on the shelf at NAPA.
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Old 12-13-2007, 04:06 AM   #10
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Quote:
Originally Posted by willwren
Look at my general chat topic. You can see one of the part numbers in a pic or two. I don't have the boxes now though, they went back with the cores.

I got both rebuilt MC'* (one 6.5, one 8") for 145 including core. 100 for the pair after the cores went back. In stock on the shelf at NAPA.
6.5 and 8"? There are two sizes? Whats the difference?

I assume the 6.5 and 8 refer to the length of the MC body.

BKNJ
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