Mongo

Thanks! Good luck to you Fred! I'm sure you will be fine!

On a side note, I noticed no matter how much I flushed, the ATE Blue permanently stained my reservoir that color.

giddyupp

You know I could have sworn I saw Mrmerlin and others recommend going front to back on previous post. Can someone please verify

giddyupp

The link is here

https://rennlist.com/forums/928-foru...-bleeding.html


About halfway down the page

Mrmerlin

its in the WSM, for the 928
LF RF LR RR

Mongo

Follow the WSM guys. I may have did it *** backwards but it's working fine now. I'll leave it alone for another 2 or 3 years...

dr bob

With the pressure bleeder, It's OK to follow normal convention of furthest-to-closest. When pedal-bleeding, the order really needs to follow the WSM recommendation. The rear of the MC is for the front brakes, and the front piston (rear brakes) depends on no air in the rear piston cavity to work right.

FredR

Bob,

An interesting post. I had this kind of dilemma some years ago when I first did my own brake fluid change out and more to the point after the local shop changed my brake lines for stainless Goodridge type and I gave the agents 3 litres of ATE blue and they used the damned lot! The way they went about it you would have thought brake fluid was free and I had to import that stuff that I purchased in expectation of three changes! I had the impression that they started at the rear and moved to the front and wondered if that might explain the huge [to my way of thinking] consumption.

When I eventually did my first change out I figured that since it was a flush rather than a "bleed the system" thing it probably did not matter and I reasoned it was better to flush out the bigger volume first. For bleeding a system that is not flooded I reasoned [rightly or wrongly] that the WSM says what it does to stop the seal from blowing out if there is no hydraulic lock on the front brake circuit [liquid being non compressible]. If that premise does in fact hold true then if I was using the pressure bleeder on a non bled system I would still use the WSM sequence even if I were using the pressure bleeder.

If it is a simple change of new to old fluid with the Power bleeder then in theory it should not matter which end is done first and thus a few days ago I went from the rear but then I saw air and got somewhat concerned as I did not expect that to happen. Thus my apprehension as to whether I dropped a clanger for whatever reason.

dr bob

Inside the master cylinder, the front piston (for the rear brakes) is functionally pushed forward by hydraulic pressure from the rear piston, which is connected mechanically to the pedal and booster. This is to balance the braking force correctly. In front brake failure, the rear piston will mechanically contact and push on the front piston, maintaining rear braking but at a lower pedal position. Similarly, a rear brake failure allows the front piston forward to the mechanical end of the master cylinder, and a second pump is often needed to gain enough pedal for a full-braking stop especially if the bleeding has not been done correctly. There's a thin washer check valve between the two sections of the piston to make this play as described. Getting the pieces all assembled correctly is a bit of a challenge without damaging the seals, and may be why we no longer see rebuild kits for our master cylinders.

The classic method of bleeding from longest circuit to shortest predates dual-circuit brakes. In single-circuit cars, getting all the old fluid out required this method, else there was risk of leaving a small slug of old contaminated fluid where there were T's to individual wheels. In dual circuit cars, we still follow the longest-to-shortest method but segregate the process by the master cylinder circuits and their pressure order. Flushing the rear of the MC first (front brakes circuit on later 928's) is needed only because fluid from that circuit passes forward to push through to that front piston check valve. Else it would make no difference front vs. rear while pedal bleeding. Flushing with the power bleeder leaves both systems open to the reservoir with relaxed pedal. Fluid forward of the rear piston cup is pushed out through the calipers, and the check valve allows fluid to pass backwards to the rear piston area and clear any contaminated fluid when front calipers are open. Note that the front piston has no such relief path, so must be cleared of contaminated fluid and any air via the dedicated bleeder port at the top front of the master cylinder.

The workshop manual method has no consideration for corrosion in the bottoms of the MC bore areas for each piston. Stan (Mrmerlin) and I discussed this problem over a decade ago, and added the recommendation that you try not to push the pedal to the floor while pedal-bleeding, as the cups risk damage when passed over any corroded sections. This is particularly important in cars that may not have enjoyed regular fluid replacement at any time during their lives, as corrosion damage is forever once it happens. Using a the little Motiv power bleeder saves that worry since you aren't moving the pedal. It doesn't remedy any existing corrosion damage, but does limit the possible damage to piston seals by keeping them clear of possible corrosion damage at the bottom of each MC bore section.


Note that the described order of flushing is applicable to the later cars. Early cars, with dual-diagonal braking, get a different order. Visit the WSM for that detail. If you have two brake light switches at the master cylinder, you have a dual-diagonal brake system. Later cars have a single switch at the top of the brake pedal, inside the cabin.