brakes slowly self engage after 20 mins driving

freeform · Post by **freeform** » 30 Jul 2018, 13:10

I have a very old 1990 Volvo 740 SW Turbo with ABS…
(also called a 2.3L L4 SOHC 8V TURBO).

It generally drives great,
the problem is after driving for 15 mins or so and after things warm up
the brakes start to activate on their own (argh)!
Only slightly at first but gradually more and more firmly until I can’t drive it any more with all the resistance,
tapping the brakes sometimes releases their grip with a snap.

I heard from a local Volvo mechanic that it probably needs 1 of the following,
(I guess I’ll try 1 at a time, probably start with the booster.)

+ Brake Booster - I think it’s this?: https://www.rockauto.com/en/catalog/vol ... oster,1884

+ ABS Hydraulic unit - on device is written Bosch part # 0 265 200 054
I can’t find it in rockauto.com's catalog but should be a fairly common item.
Crazy expensive new so hope to find a used one for $130 or less , gulp.
Might try eBay for my 1st time... I live in California, anyone know where to get one?

(Fwiw, it’s our 2nd car used only to drive within a 5 mile radius a few times a week...
it's nearing it's end of life and at 200k miles not worth a lot of repair investment.)
Thanks for any tips!
Rand

Post by **jimmy57** » 30 Jul 2018, 13:44

Take wrench(es) that allow the master cylinder to be loosened up from the brake booster. When it does it, stop and loosen the master cylinder from booster 1-2mm from booster and see if the car now rolls easily. The brake booster is the leading cause of all wheel brake apply, especially on the car in question.

I'm assuming you know it is all wheel apply and not just one sticking caliper or blocked hose at one wheel.

freeform · Post by **freeform** » 30 Jul 2018, 13:53

THANKS so much for reply!!!!
(I've taken this to 3 mechanics and had 3 different replies.
last 1 said he put it on lift and it was all wheels.)

Hmmm, a few questions, I hope you don't mind..
+ Have you heard or seen this behavior before, odd it only does it after warming up?
+ Will this loosening be safe?..
+ Will it 'fix' or just indicate what's wrong?
+ Do you imagine it has nothing to do with the ABS Hydraulic unit?

THX AGAIN!

Post by **jimmy57** » 30 Jul 2018, 13:59

I've seen it many times on many makes.

Loosening the master cylinder from booster is a test to see if brakes release. It is safe to do that and then put it in gear and see if it now rolls and coasts as normal.

Just a test. If the booster is not working right then it needs to be replaced.

I have only seen one circuit ABS brake sticking (your car has 3 hydraulic circuits with the the two rears supplied as one circuit and would stick on at one time). I really do not expect it is ABS related.

freeform · Post by **freeform** » 30 Jul 2018, 14:06

Sweet!
The booster is not very expensive..
but the ABS Hydraulic unit is very expensive.

I'll probably buy one of these from rockauto.com
https://www.rockauto.com/en/catalog/vol ... oster,1884
I think this is the right one for my model??
https://www.rockauto.com/en/moreinfo.ph ... 414&jsn=10

THX!

freeform · Post by **freeform** » 30 Jul 2018, 14:12

Re-reading your advise, sorry but just to be clear...

So you're saying by loosening the master cylinder from booster and it rolls AND brakes normally then it's the booster that needs replacing?
BUT if it still has the problem with brakes active then it's not the booster.. or still could be?
Sorry again, I guess I'm confused what each result would mean.
Thx!!

93Regina · Post by **93Regina** » 30 Jul 2018, 20:00

freeform wrote: ↑30 Jul 2018, 14:12but just to be clear...

Logic 101 - Something must be depressing the Master Cylinder to activate brakes.

Two Options

1. Driver's Foot

2. Brake Booster

If vacuum is removed from Brake Booster, which means more pedal force is required to stop, and it works fine....then what will you replace?

93Regina · Post by **93Regina** » 30 Jul 2018, 20:04

jimmy57 wrote: ↑30 Jul 2018, 13:59I have only seen one circuit ABS brake sticking

Sidebar - Solving Brake Lockup on Honda/Acura Boosters

lummert · Post by **lummert** » 31 Jul 2018, 14:04

The ABS can be disabled by disconnecting both front sensors. Do this at your own discretion depending on your ability to drive with disabled ABS. Another thought: brake hoses can break down internally resulting in check-valve like effect of not releasing the calipers. it would be unusual, but not impossible, for this to happen to all 4 brakes at the same time.

freeform · Post by **freeform** » 31 Jul 2018, 14:09

thx..
my last mechanic said this too & pulled the square mutipin thingy..
it's fine for me since I'm a good driver & only driving very local at traffic speeds in LA..
so no rain (usually) or snow.