Pajero braking problem - hard pedal

[Cameron]

Hi guys,
I've had a search around but can't find anything pertaining to my issue....

I've got a 1993 pajero swb 3.0v6, manual, and abs.
Anyway, I bought this thing about 3 weeks ago. After a few days of ownership, I noticed that intermittently, my brake pedal would go hard, and pulsate under my foot very slightly. Later it would begin to make a squeaking noise when foot on brake, and pedal was hard.
After a few seconds, the pedal would go back to normal and braking would be fine.
I figured it was the booster, as there was often some engine surge at constant throttle, and if you put your foot on the clutch when the brake pedal went hard or the 'surge' presented itself, the engine would idle at about 3000rpm, and the brake pedal would again be hard. when the pedal returned to normal, the revs would drop, and all was well...sure sign of a vaccuum leak, I thought.
Weirdly enough, though, this problem would only happen at open road speeds most of the time.

We threw a vac gauge on where the 4x4 solenoids tee off next to the booster. Engine vac didn't change measureably when this problem came up. Again, I figured it was sucking past a small hole/tear in the vac booster diaphragm.
The hoses *seem* to be in good condition to the booster. the check valve also seems to function correctly. I replaced the jap pinch style hose clips with proper ones, done up nice and tight.

I got a secondhand booster from a wreckers and fitted that last night. The problem's still there, although the engine doesn't get the whole surge thing anymore (at least I don't really notice it this time round).
The vehicle has ABS. the abs pump still functions (I know this from a test I did with the whole 'emergency stop on a wet road', to make sure it still worked.) and the light hasn't come on so far.

I've got another hose that hooks to the booster. I reckon I'll try fitting that next and see what happens. I'm now no longer sure that it was the fault of the booster or hoses, now, though but that would have explained the 'high idle' symptom.
Basically, i'm at a loss where to go from there.

None of the other vacuum hoses on the engine are damaged, and the fact that this can't be simulated without the paj going at least 60-70kph, I'm thinking it may be something to do with the abs pump/system?
I don't have a lot of experience with ABS, is one of the main problems.
Apparently it had new pads about a year ago. I've had to sort out a failed 80s bmw abs system a few years ago for a friend who did his pads and pushed the pistons back in without opening the bleed screw. Is this a potential issue for a pajero or is that just the archaic BMW bosch system in that case?
It's stock standard, too. no liftkits, no mods, stock as a rock.

so where do I go from here?
Is there merit in unplugging the ABS pump and seeing if that eliminates the problem? is it likely to be a valving issue in the ABS system and not related directly the the electrics of the pump?

At this stage I don't think it's going to be an issue 'further down the line' (calipers etc). there's no dragging of the brakes, wapred calipers, stuck slides, or anything like that.

Any help would be great.
cheers
cam

[flyingbrick]

hey dude.

have you tried pulling the fuse for your ABS?

-Nathan

[Cameron]

Was thinking of doing that to see what happens.
Might have a go at that after work.
thankfully there's two of these at pick a part hah. God knows what a wrecker would charge for an abs unit.

[pjb]

Interesting. I've got a 3.5 with ABS. I've occasionally noticed the very same thing: jumped on the brakes at open road speed and noticed a firm pedal (slight pulsating) and not much braking, then momentarilly later, pedal sinks down, and much more anchors. It's only ever been occasional, and only ever lasts about a second...

If you make any discoveries, I'd be very interested to hear about them.

[Cameron]

yeah I've spent a fair bit of time on the old googley mobile. seems only people report problems with the hydraulically boosted ones.

So, looks like tonight I'll try:

'disabling' the abs system, then road test

removing pcv valve from system (although I don't think this'll make any difference...) - then road test

replacing rubber hoses (same deal as pcv valve in my eyes... gotta cover all bases) - then road test

should've stuck to Citroens, maybe? They're way easier to fix... just fix everything....

[Cameron]

alrighty.....
I might add that the revs no longer build and the engine doesn't get a surge thing going on when this problem's about. I'm not sure if they're linked or what the story is but, still, it's pretty weird.
Anyways, I unplugged both plugs from the abs pump before I left work tonight. drove halfway home mashing brakes at random - no issue.
Stopped on the side of the road, whacked plugs back on, within 100m of getting up to speed again I tapped the brake and boom, problem's back.
It would seem it's something in the abs system that's at fault.
I'm thinking the vac booster was a bit toasted, given the revs and surge thing disappearing since chaning the vaccuum booster.
I'm now thinking my plan of attack will be to bleed the crap out of the brakes. This will eliminate there being any issues with air in the system.
From there, if the problem is still about, I'll probably change the pump. While bleeding the brakes I'll check and clean the connections and faces of the abs sensors. Is there a way of checking these with a multimeter? any way to 'check the fault codes' on the abs system or is it a bit old for that tech?

could be an easy fix.

[pjb]

Facinating...
You'd think if there was a problem with the ABS system you might see the ABS light on at least occasionally...

[Cameron]

I would've thought so too.

Had a bit more of a bugger round on google. Few people seem to have a problem in volvos and saabs with wheel sensors giving similar issues.
Also, across a few jap brands as well, people report pumps giving up the ghost with pad swaps being done without pushing the pistons in calipers back with bleeders open so it doesn't backpressure the pump, which I've seen on older abs systems.
So it could still be the pump.
Need to do a bit more reading i reckon.

[Raiknn]

Had a similar issue with mine. Was damn scary on failed climbs etc because the ABS would just firm up, pulse and send you backwards rapidly. I ended up disconnecting it and havent had it connected for about 2 years now.

[Cameron]

yeah its a bit of a worry.....

trying to find time to talk to a few brake guys now. hah.