Brake system drain

[KING46Calais V]

Yes, you will need to bleed the ABS module.
Do you need to change the master cylinder?

And I'll need a GDS to do that?

Yes I want to move from the 25mm to the 27mm master

 

[RevNev]

That was the word on the street when abs first came out 30 years ago but I have heard several people say since then that it was just a scare campaign. As long as you keep the master cylinder above empty there isn't a problem.

Correct! If you don't let the master cylinder reservoir run out of fluid and particularly whilst pumping the pedal to suck air into the ABS pump, there'll generally be no problem bleeding the system. Blocking the brake lines to prevent the master cylinder draining out when removing calipers is also essential. If you change the master cylinder, you'll need to cycle the ABS pump with a scan tool whilst bleeding.

 

[lmoengnr]

And I'll need a GDS to do that?

Yes I want to move from the 25mm to the 27mm master

If you're using a Bosch MDI or clone type tool, you should already have access to GDS.
There are other scan tools available that should be able to operate the ABS module to bleed it.

 

[RevNev]

Yes I want to move from the 25mm to the 27mm master

Why? Here's the original master cylinder fitted to a Gen F2 LSA Clubsport with HSV AP 4 spot calipers.

 

[NJD-1992]

If you disconnect the calipers or let air into the system (too low master cylinder etc) then you do need to do the 'special drain' which is using the scan tool to cycle the ABS pump.
Otherwise as others have said as normal.

I replaced all my calipers and hoses, didn't bleed the ABS pump but didn't see a need too?

Only replaced one caliper at a time, plugged the hose fittings with bolts to prevent as much fluid as possible leaking when lines disconnected, and also pre-filled the calipers with fluid to prevent excessive fluid pockets in the system. Kept the resi topped up and bled each caliper as I went along before moving to the next one.

5000km later, no issues with braking that I can feel? (Including repeated HARD stops from 110KM/h out on the remote freeways)

 

[RevNev]

I replaced all my calipers and hoses, didn't bleed the ABS pump but didn't see a need too?

If you unbolt a brake line at the caliper, let the master cylinder drain out and then pump the pedal. That's when you'll suck air into the ABS pump. and you'll end up with spongy brake pedal without cycling the ABS pump.

If you don't let the master cylinder empty out on a VF when unbolting caliper brake lines, the brakes will generally bleed up fine without cycling the ABS pump. The same applies when bleeding brakes, don't let the master cylinder run dry!

 

[NJD-1992]

If you unbolt a brake line at the caliper, let the master cylinder drain out and then pump the pedal. That's when you'll suck air into the ABS pump. and you'll end up with spongy brake pedal without cycling the ABS pump.

If you don't let the master cylinder empty out on a VF when unbolting caliper brake lines, the brakes will generally bleed up fine without cycling the ABS pump. The same applies when bleeding brakes, don't let the master cylinder run dry!

Yep, learnt that many many years ago the first time I tried bleeding mountain bike brakes!

I was ridiculously careful to make sure the master cylinder didn't empty, making sure it was almost overflowing. Undone the calipers and hose from the metal fittings on the chassis, but only 1 at a time and bled each caliper individual furthers to nearest

 

[KING46Calais V]

Why? Here's the original master cylinder fitted to a Gen F2 LSA Clubsport with HSV AP 4 spot calipers.

View attachment 264882

I do prefer a firm pedal with least travel. But I suppose I should ask for anyone that has gone from 25 to 27mm and what their experience was difference wise, if any. Now your photo has got me thinking.
Maybe Holden stopped supplying 25mm because the effort to get good clamping was too great.....
(Please lets not get into the debate of engineering, standards, being tight etc, just info re experience going from 25 to 27 or vice versa)

 

[NJD-1992]

(Please lets not get into the debate of engineering, standards, being tight etc, just info re experience going from 25 to 27 or vice versa)

 

[RevNev]

Maybe Holden stopped supplying 25mm because the effort to get good clamping was too great.....
(Please lets not get into the debate of engineering, standards, being tight etc, just info re experience going from 25 to 27 or vice versa)

Going from a 25mm to 27mm master cylinder, you'll press the brake pedal harder for the same braking force, but the pedal travel will be longer.

Some of the late production MY17 Redline's (my ute included), were factory fitted with the 25mm master cylinder. Quite a few people who bought 27mm master cylinders by part number from 2017, were 25mm master cylinders with the 27mm part number on the box. I remember Harrop did the hydraulic calculation for their 6-piston front and 4-piston rear kits and found the 25mm master cylinder was more than adequate fluid displacement. I don't know what HSV used with 6-piston front calipers, but they retained the stock 25mm with their 4-piston calipers.