Any tips for brake booster R&R?

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

Planning on replacing the brake booster this weekend. The FSM suggests that a number of brake lines should be removed from various places in order to make this happen; I'd prefer not to have to do that since I don't have the DRB-III tool needed to bleed the lines.

Is there a way that this can be done without having to undo the brake lines? It seems like it may be, but I'd like to hear from folks who have been there before.

flash7210 · **Posted:** Sun Aug 02, 2020 5:00 pm

If you haven’t figured this out yet then I’d say you can try unbolting the master cylinder and bending it and the lines forward to see if that will give you enough room.
Gently of course.

If you do end up completely removing the master cylinder then do all your brake bleeding with the key off and/or the ABS fuse removed.
And if you can, do a reverse bleed by pumping fluid into the calipers, thus pushing air out into the master cylinder reservoir.

Why do you want to replace the brake booster?

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

flash7210 wrote:

If you haven’t figured this out yet then I’d say you can try unbolting the master cylinder and bending it and the lines forward to see if that will give you enough room.
Gently of course.

Yep, this was about the best idea I had come up with. I'll give it a shot when I have the time to do the job; this past weekend ended up being not quite as free as I'd expected.

Quote:

If you do end up completely removing the master cylinder then do all your brake bleeding with the key off and/or the ABS fuse removed.
And if you can, do a reverse bleed by pumping fluid into the calipers, thus pushing air out into the master cylinder reservoir.

Got it - it sounds like that would negate the need for the DRB-III.

Quote:

Why do you want to replace the brake booster?

It's failing the vacuum hold test, and I'm noticing more pedal effort being required to bring the KJ down to a stop. Brake fluid levels are constant, so suspect a failing booster rather than hydraulics. Pads & discs are new enough (18 months) that I'm not concerned about them, and appear to have worn evenly in both cases with good meat still remaining on the pads.

WWDiesel · **Posted:** Mon Aug 03, 2020 2:56 pm

When I replaced my calipers and rubber brake lines, I used the a pressure bleeder by Motive Products.
It worked like a champ, no issues with anti-lock controller.
There are some other power bleeder brands available that I am sure will work equally well.
Bleed them with engine OFF, and key OFF.

layback40 · **Posted:** Mon Aug 03, 2020 7:22 pm

casm wrote:

It's failing the vacuum hold test, and I'm noticing more pedal effort being required to bring the KJ down to a stop. Brake fluid levels are constant, so suspect a failing booster rather than hydraulics. Pads & discs are new enough (18 months) that I'm not concerned about them, and appear to have worn evenly in both cases with good meat still remaining on the pads.

Classic signs of a failed diaphragm. What often happens is you get oil fumes in the booster & they destroy the diaphragm. This can happen on cool down when there is a leaky non return valve in the vacuum line.

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

WWDiesel wrote:

When I replaced my calipers and rubber brake lines, I used the a pressure bleeder by Motive Products.
It worked like a champ, no issues with anti-lock controller.
There are some other power bleeder brands available that I am sure will work equally well.
Bleed them with engine OFF, and key OFF.

I looked that bleeder up, and I'll just say that I like it a lot. Thanks for the recommendation; definitely planning on adding it to the arsenal.

layback40 wrote:

Classic signs of a failed diaphragm. What often happens is you get oil fumes in the booster & they destroy the diaphragm. This can happen on cool down when there is a leaky non return valve in the vacuum line.

Yep, been through this many times before and the symptoms fit. I did try swapping out just the check valve on the booster, and while that helped a bit, the problem continues. There are also three or four check valves in the vacuum path between the booster and the turbo vane gubbins (I'm including the one behind the glovebox for the A/C in that count) that are planned for replacement just to give my OCD some satisfaction.

layback40 · **Posted:** Tue Aug 04, 2020 7:11 pm

casm wrote:

layback40 wrote:

Classic signs of a failed diaphragm. What often happens is you get oil fumes in the booster & they destroy the diaphragm. This can happen on cool down when there is a leaky non return valve in the vacuum line.

Yep, been through this many times before and the symptoms fit. I did try swapping out just the check valve on the booster, and while that helped a bit, the problem continues. There are also three or four check valves in the vacuum path between the booster and the turbo vane gubbins (I'm including the one behind the glovebox for the A/C in that count) that are planned for replacement just to give my OCD some satisfaction.

What happens is, when you turn the engine off, the vac pump that normally discharges into the engine, allows fumes to back flow all the way to the booster. It becomes a big catch can. You may also have issues with other diaphragms. None of them like engine oil.

casm · **Joined:** Sun Jan 29, 2006 5:05 pm **Posts:** 694

Followup:

Booster was replaced a week ago; have about 500 miles on it. As expected, the brakes are now far more effective than they had been for some time.

As a bonus (which I'll admit I was hoping for), the sluggishness under about 1600rpm is now gone and power overall is much closer to where it should be. When the brakes started requiring noticeably more effort than they had in the past, the penny dropped with respect to what might be causing the off-the-line acceleration issue since a dying brake booster would cause a massive vacuum leak.

Fuel economy is slightly improved - perhaps 0.5mpg. However, it's still not as good as it should be and soot remains unchanged (read: too much). I'll have to chase that one down, but at least one major issue has been ruled out.

Any tips for brake booster R&R?

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