Creeping brake pedal. No leaks, not master cylinder....

The car in question is my 2003 Vauxhall Vectra. It's the petrol 3.2 V6. It's not 'diesel creep' in that it's not a diesel.

The brake pedal is a bit soft. When stationary, if you apply a lot of pressure (brace your back against the seat and heave on it) the pedal will creep towards the floor. This is pressure above what you'd use to skid to an ABS stop whilst driving. Nonetheless, I don't think it should do it. It only does it with the engine running (servo assist).

When I hook my Verus up to it, and (via the ABS menu, functional tests sub-menu) get it to test run the ABS pump for two seconds, it temporarily fixes the problem. Leave the car for an hour and it's back to where we started.

It's recently had two new rear calipers, two rear flexi-hoses, rear pads and discs and a centre brake pipe replaced. It then got a flush of the braking system via a pressure-bleeder and filled with DOT5.1 fluid. The history was an advisory on the last MOT about a corroded brake pipe. When I went to bleed the system after the work the alloy caliper threads decided they'd rather attach to the bleed nipple than remain as part of the calipers, hence two new calipers. I changed the flexi-hoses on the 'whilst I've got the calipers off, I might as well' logic, and the brake fluid flush was just intended as normal maintenance. Pads and discs are genuine Vauxhall, from a main dealer's parts counter. Calipers and hoses are Pagid.

Until I had a play with the Verus today, I'd resigned myself to ordering a new master cylinder, but in light of how it behaved immediately after I'm now a bit lost. It had a rock solid pedal (to the point I was hurting my foot through a pair of steel mid-soled work boots) I've concluded the master cylinder is therefore not to blame.

What's happening? Anyone know?

Cheers,

Paul

Sinking pedal with no trace of leak is nearly always a master cylinder issues, usually it shows up as very slowly sinking pedal when a fairly gentle but constant pressure is kept on the pedal.
However Brake master cylinder issues after bleeding is a widely reported issue on Vauxhall/Opel models, it has been called by some "seal reversal".

There are two simple tests for a master cylinder issue. The first is simply fill the cylinder up to the brim and observe what happens to the fluid level while and assistant applies and holds pressure on the brake pedal, this can be quite a difficult test to get a definitive result from. The second test needs more preparation but gives a 100% reliable result, basically disconnect the master cylinder and replace the brake unions with bleed nipples bleed out any air and test the pedal. With a ABS mastercylinder you may need to make up special short pipes with reducer unions.

Tip to reduce fluid loss while working either prop the pedal fully down while doing this, or use the cling film trick fill the fluid level up until it is to the very brim and stretch cling film across the meniscus and make an air tight seal round the neck with an elastic band,

The reason I didn't think it's the master cylinder is that immediately after I've run the ABS pump via the Verus (and for the next five minutes of driving), the pedal's rock solid, with absolutely no creep, even under ridiculous pressure on the brake pedal.

Am I misinterpreting something?

Cheers,

Paul

I agree with Andy. Although I've never worked on a Vauxhall, nearly every creeping pedal I've encountered was due to the master. Unless there's air being drawn in somewhere at a loose union...

Sounds familiar to the fords, the abs valve Block will cause the pedal to drop, there's has rubber plugs were you can place pins in to see if valves are moving when the shouldn't. Can't recall proper names for the unit, but I've done a few recent with a fading pedal, and both (ford)were that abs valve block,not sure if yours is setup the same though

I have ran into the same complaint on several occasions and have been burnt in the past. You live and learn.

Now I take the vehicle out on a dirt road and go into full on ABS assisted braking, try this 2-3 times. This will force the modulator to work and build internal pressure. If the problem temporarily goes away and you get hard pedal back, it is safe to say the ABS/ESP module is at fault.

By engaging the ABS/ESP unit via Verus, you have ruled out a faulty MC by completing this test.

I have never taken them apart to see why exactly they act this way, worn seals/valves on solenoids, maybe.

Did you ever sort this out?
No pressure, just curious.

I remember the first time I did brakes on a 93 gm trk, had no idea I was supposed to bleed the abs module separate. Had a goofy pedal for a couple weeks