camboinc

Guys, I am in urgent need of assistance on this one. My shark won't move. It's like you're stepping on the brake pedal hard when in gear.

The back story: my ABS never really worked on the car. Murf can attest to that! Prior to this, the car would brake like a non-ABS car. Mash the pedal and the wheels would lock up. Skid city.

Dave K and I just pulled the dash and replaced HVAC diaphragms. When we put the car back together, everything seemed to work okay. Took the car to my stereo guru and dropped in a new system. The car seemed to be fine. Within 100 miles or so of the stereo install, I was driving along and you could feel the brakes being applied. However, I was not touching the brake pedal! Fortunately, I was less than a mile from home. I continued to drive and it felt like more brake pressure was there. By the time I got in the driveway, all four brakes were smoking and the car would barely move.

I would assume that the ABS is applying the pressure but it won't let up. When I start the car and attempt to move, the brakes are engaged enough that I can't roll or move freely in neutral. It also feels like there is not much brake pedal travel.


What could be the culprit here? ABS sensor or system? Air in brake lines? Bad brake booster or reservoir? Some wire that was disconnected ?

I continue to get zero reprieve from the 928 gods. Please help keep me in this car with some easy-ish fix.

Rhudy out.

Mrmerlin

disconnect the power booster hose and plug it,
see if the brakes release.
check the vacuum lines they may not be routed correct

you more than likely need to bleed the brakes as the fluid is boiling and putting the brakes on, this also may include replacing the MC and or booster,
the brake lines may also be damaged if they were pinched,
new stainless lines could be fitted,
the brake bias valve may also be damaged this requires replacement its screwed into the ABS unit
you dont have any issues with the ABS causing the sticking brakes.

NOTE if you recently swapped out the brake fluid with another brand you could have caused the seals to swell this requires a new MC and lines and possibly rebuilding the calipers
As always I suggest to use ATE super blue fluid it wont damage the seals

James Bailey

ABS NEVER pushes adds brake pressure it ONLY subtracts.....More likely is the booster is applying the brakes. Had it happen to me on an 86, by disconnecting the vacuum to the booster you have no such help. You then have very hard to push manual brakes.
Sometimes you can adjust the free play on the pedal other times you must replace the booster.

camboinc

thanks guys. i'll do a little fiddling tomorrow and hope for an easy fix. :-)

namasgt

Sounds like the brake booster like Jim has said.

davek9

Thanks for the assist, Brian just sent me a text on this one, the brake fulid was changes last summer with then super blue ATE, IIRC he's got the SS lines already installed.
The Booster makes a great first choice as the Vacuum is now holding.

So what goes wrong with the booster ?

Thanks again for the help!

DaveK

James Bailey

There is an internal trigger/switch which regulates the boost assist pressure seems it can fail and apply the brakes and then with heat and expansion the brakes start locking up as described. Pull the vacuum feed and no "assist", no locked brakes.

dr bob

I'd start with pedal freeplay. Easy to check, and adjust in the drivers footwell.

davek9

Thank's Jim! I was wondering if a mighty vac test would help, so the booster is most likely holding vacuum and the internal "switch" is the issue.

dr bob, this adjustment was not changed is it something that could have changed on it's own?
This car has made trips to Green Bay and SITM w/o this issue, although it's been off the road for a year.

edit: Brian is pulling the "main" booster vacuum line and is going to text me back, w/ results.

Thank you all for the quick replies.

Dave K

Lizard928

The freeplay of the MC should be checked like Dr. Bob suggests.

James Bailey

The one I played with I adjusted freeplay and it seemed to help for a while then it began self braking again....my theory the switch/valve got worse but who knows ????

the flyin' scotsman

the booster would be what Id check........given its bad its probably taken up all the free play and more.

Once the problem is corrected the brake fluid should be flushed and bled..........its been boiled !!!

also after this isuue is corrected the ABS can be sorted.

davek9

Thank you Colin and Malcolm for adding to this, today is the first I heard that the Anti-lock system was in-op, yep thats next.

Dave K

Lizard928

I've seen more and more ABS system failures over the past couple years.

If the light comes on the moment car is started, then its relays, or a bad pump.
If the light comes on once the car has moved then its sensors. This could be even just a build up of metal shavings on the sensor itself, so first is to remove all sensors and clean off metal shavings.

davek9

Any most common causes, other than bad wheel wiring?

edit: Good first steps, thanks!