Poor Braking with Power Brakes Car (Page 1/1)
72and86 APR 23, 11:54 AM
Doing a light restoration on a 1986 SE V6. The back brakes have been completely replaced and work fine. New everything including brake lines. (Parking brake is not locked up) Front brakes look ok and not serviced. Calipers working fine, great pads.
PO replaced lots of brake parts.
The problem is it feels like there's no (or very little) Power Brake assist when braking. There's plenty of vacuum getting to booster. And with car off , the booster hisses nice when line is disconnected. But, Brake Booster has been changed. No effect. Still brakes like a manual brake car. My mechanic buddy says the only thing left is proportioning value. Thoughts?

Thanks
Steve
olejoedad APR 23, 12:03 PM
The bias valve seldom goes bad, bit it is a possibility.

Does the spool in the valve move?

Is the proper master cylinder on the car?
theogre APR 23, 01:52 PM
New Booster or Used booster to replace old Booster?
Vac Booster is Dual Diaphragm type and 1 section goes bad causing higher pedal effort and less brakes. Pedal Effort is noticeable Worse but not as bad as Engine Off and No Vac. Used Booster could have same problem.

84-86 Booster Vac line is 3/8" size (MAYBE metric or not. Some things are Not metric.) and have a filter on rear firewall that can be block w/ crap. Check the filter to see if flows air relatively easy.
If the filter is blocked, discon line from booster, leave filter off and blow compressed air thru cabin section to blow out crap Before you try to run the car w/o the filter.

Combi valve and MC are Aluminum are many install line nuts too tight and crush the line hole closed.


quote
Originally posted by olejoedad:
Is the proper master cylinder on the car?

Good Q because many put a "Big MC" crap from TFS etc w/ stock brakes that kills brake output at the calipers.

"Good Pads" can be Wrong Pads like try to use Race Pads on Street and Pads never get hot enough to work right.

------------------
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(Jurassic Park)

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Dennis LaGrua APR 23, 01:56 PM
Poor braking action can be caused by:
Warped or worn rotors
Contaminated brake fluid that may have never been changed
Air in the system or one that has been bled incorrectly.
a defective brake booster

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
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edfiero APR 28, 10:24 AM
You mentioned new brake lines.... are you talking about the hard lines from the master cylinder or the flex lines to the calipers?
Flex lines do go bad even when looking fine on the outside.
I'd suggest bleeding the brakes, the old fashion way with a helper pumping the brake with you cracking open the bleed screw. You want to make sure you get a similar amount of fluid coming out of each caliper.
72and86 APR 28, 10:50 AM

quote
Originally posted by edfiero:

You mentioned new brake lines.... are you talking about the hard lines from the master cylinder or the flex lines to the calipers?
Flex lines do go bad even when looking fine on the outside.
I'd suggest bleeding the brakes, the old fashion way with a helper pumping the brake with you cracking open the bleed screw. You want to make sure you get a similar amount of fluid coming out of each caliper.



Both types were replaced, MC and flex. We did bleed the old fashion way. And my mechanic says its not the fluid because we have a nice hard brake pedal. If it was the fluid or air in the lines it would be mushy. which its not.
Mike in Sydney APR 28, 07:37 PM

quote
Originally posted by Dennis LaGrua:

Poor braking action can be caused by:
Warped or worn rotors
Contaminated brake fluid that may have never been changed
Air in the system or one that has been bled incorrectly.
a defective brake booster




I vote for a defective brake booster. If that's the case, now may be the time to upgrade to an S10 booster. Also, check your pads. I like Wagner ThermoQuiet pads. My seat-of-the-pants braking dyno tells me that the braking performance is much improved over the stock system.

Be aware, YRMV. Of course, I did the booster upgrade and pad change on one of my '86 GT's when I did overhauled the brakes. My other '86 GT has a purely stock system in good nick and the one with the S10 booster and ThermoQuiet pads stops much quicker with excellent control.