Brake pressure fault

[Terry82]

Hi lads.
I’ve been having trouble bleeding my brakes after I fitted a new rear caliper brake piston.
I use a pressure bleeding system and I can get a good stiff pedal after bleeding all four calipers.
But as soon as I turn the ignition on and start the engine, the brake pedal goes soft, almost to the floor.
While holding the brake pedal down the engine starts to hesitate and hunt a little.
I’ve gone round bleeding the system several times to no joy.
I’m lost been wreaking my head to try and figure out what I’ve done wrong...

[titchy]

try bleeding the longest line first n/sr then osr then n/s front don't press the pedal to fast make sure it goes to the floor does the pedal stay firm with engine off

[Terry82]

Thanks.
Yes the pedal stays firm with the engine off.

[titchy]

check for  servo hose leak

[titchy]

Or brake booster valve there is a non return valve which can fail to the r/h when looking from the front below inlet man round with 3 pipes

[Terry82]

Thanks bud

[LC5F]

Did the brake fluid run low whilst bleeding?
Sounds like air in master cylinder.

[Terry82]

I use a sealey brake bleeding kit, it’s a big bottle with a pump on top of it to it attaches to the res and pressurises the brake lines.
Never had any issues before. Only recently fitted braided lines with out a problem bleeding the system.
The brake fluid ran low in the sealey brake bleeding bottle and sent air under pressure about 8psi through the res at the master cylinder. The air bubbles made all the way to the caliper I was bleeding at the time, almost instantly once the kit ran dry of brake fluid.
Anyway I haven’t had a chance to work on it again yet but I’ll give it a go at the weekend. A mate has a gti that’s been parked up for ages I might be able to swop some parts off that, if it is the brake servo or pressure release valve.

[maxamplitude10]

Needs bleeding again sounds like definite air in the system, when the system pressurises the air will compress and the pedal will travel further?

Max

[pudding]

I just get VW to do the bleeding/flushing after changing calipers.  They do it with the ABS pump activated which evacuates all of the air and also any dirty oil gathered around the pump pistons.  Always get a perfect pedal after they do it.  I think it's £60 every 2 years as a regular service or a one off, so not too bad.

[LC5F]

The brake fluid ran low in the sealey brake bleeding bottle and sent air under pressure about 8psi through the res at the master cylinder. The air bubbles made all the way to the caliper I was bleeding at the time, almost instantly once the kit ran dry of brake fluid.

The trouble with the pressure bleeding kits is they make you complacent not checking the reservoir - done this myself on my old Mk4 ran the reservoir dry - they have a thimble for a reservoir, so really easy to do.
On my Mk4 it was hard with engine off - but pedal sank to floor with vacuum assistance - it was an air bubble in master cylinder. Mk4 is easy to bleed as it has nipples... not seen any on the Mk5, likely you will need to bleed the line at the master cylinder.