Brake vacuum issue....whats my problem? (Page 1/1)
Kitskaboodle NOV 29, 11:10 AM
Hi guys, here is the issue on my 85 GT Auto:
After a drive on the freeway, when I take the off ramp for my exit and hit the brakes, they are quite weak and spongy until I pump the pedal a few times to get the pressure back. After this they are acceptable. It does this around town as well but since in stop and go driving you are starting and stopping much more, it’s not as noticeable.

Anyways, here is a curious new update to the story.... I have distinctly noticed that when I first start my car on a cold morning, my engine goes to a good 1700-1900 rpm during the cold idle circuit. It takes a few minutes of driving for the rpm’s to go down to normal. Anyways, during this brief period of higher than normal rpm, I noticed brake pressure is much better and I don’t need to do any pumping of the brakes at this period of time.

As an FYI I replaced the booster and m/c about two years ago and this problem was not there after I replaced them. The brakes are all stock and the pads are probably about at 50%. R-brake all hooked up and working fine.
I don’t think this is an air in the brakes issue but rather a booster or check valve issue.
What tests can I perform to narrow down the problem? Your thoughts?
Thanks, Kit
skywurz NOV 29, 11:52 AM
With the pumping/spongy you are describing im going to vote bad mc. The mc is going bad and not moving fluid properly. There's the old stories of going out to the car in the morning and you foot going right to the floor because the mc went bad.

If it was a bad booster i would expect the pedal to be stiff an you to have to stand on the stiff pedal to get it to slow and stop.
DimeMachine NOV 29, 01:58 PM
Spongy. Makes me always first think of air in line. Is your brake light coming on warning of pressure differences between front and rear brake circuit?
Kitskaboodle NOV 29, 02:29 PM
No brake light warning on dash.
Also, I question why brake feel gets better just from higher engine rpm and no pumping?
Kit
DimeMachine NOV 29, 06:56 PM

quote
Originally posted by Kitskaboodle:

No brake light warning on dash.
Also, I question why brake feel gets better just from higher engine rpm and no pumping?
Kit



Hook up a vacuum gauge and see what it is at at idle and again holding it at 2000 RPM.

Did you check the vacuum filter on the firewall to make sure it is in good shape?
Kitskaboodle NOV 29, 07:09 PM
Vacuum gauge connected to what, the hose that goes to the check valve on the brake booster? I’m aware of the filter by the firewall. How do I verify it’s operation?
I do have a MightyVac bleeder/vacuum pump kit.
Unfortunately, when it comes to troubleshooting this kind of stuff I’m a rookie.
Thanks, Kit
theogre NOV 29, 08:26 PM
"Brake vacuum issue....whats my problem?"
Is Not vacuum or booster problems.
Not MC problems since you Can pump them and get pressure.


quote
Originally posted by DimeMachine:
Spongy. Makes me always first think of air in line. Is your brake light coming on warning of pressure differences between front and rear brake circuit?

Right idea but Not Air because drive on highway before happened... Likely Boiled the Brake Fluid because likely one caliper has drag pads making heat problems.

Brake Warning light often Won't light for air or "steam" in the system for several reasons even when combi valve and switch is 100% good.

Wet fluid can Boil ~ same as Water, ~ 212°F/100°C.
So if a pad drags even lightly on highway or anywhere can boil weak fluid.
Flushing out wet fluid isn't same as bleeding because even when all calipers are compressed, "Fiero type" rear pistons still have a lot of old fluid.

might get "lucky"
Make sure bleeder screw works
then either...
1.
Remove bleeder and find very thin tube to push in bleeder hole but Not seal the hole.
Flip cal bleeder down and use tube to let air in and fluid "leak" out.
Do not use compress or "canned" air as often wet oily or often causes more problems beside spraying fluid isn't good.

2. This is last option because can wreck P-screw seal in the bore you don't see easy.
Loosen arm on the rear cal's.
take cal off then firmly block piston so can't push out.
take arm off the rear cal's and Carefully turn P-screw in to Cause a big "leak."
CAREFULLY turn cal to P-screw down to dump most of fluid.
May need careful use of needle nose pliers etc to get P-screw turn out again after. If still can't reach then Carefully compress the piston and watch P-screw doesn't "go sideways."

Either is goes to make a mess.

before you ask, no, front doesn't really have problems like this but only need to compress the piston to reduce fluid volume. Use DOT4 so some old fluid won't matter much.
Make sure the Sliders are lubed w/ Brake Grease.
See my Cave, Brake Service and Brake Fluid

If rims have open spaces to see the brakes...
To find pad drag problem use IR thermometer (cave link) and sim but often hard to find or cost more since CCP Virus problem. Both side of same axle should read very close to =, front can be higher or lower the rear.
Best if can drive on highway then stop somewhere safe and quick read both fronts then both rears.
Actual numbers often don't matter as long as you measure in order and same spot(s).
In this case, also try to measure inside of caliper. May need to have something to lay on and move fast.

(Any Contact temp tools are harder to read fast.)

Note... Might be possible V6 w/ crack exhaust man or missing heat shield could boil brake fluid in line across the trunk wall to left rear cal.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 11-29-2020).]