odonnell

I'll also add that I tested the check valve that plugs into the grommet on the booster and it tested 100%.

As far as brake lines, they're braided SS and only a few years old. I would expect the car to pull to 1 side under heaving braking, versus the general brake feel to be really lackluster and dangerous. Correct me if that's a poor assumption.

odonnell

Another quick question. Does the O-ring between the master cylinder and the booster play a role in the integrity of the booster vacuum? I saw some videos of people testing boosters off the car using the vacuum pump but I'm sure there are design differences.

thomasmryan

I would start capping ports to eliminate things.

Boosters at rest, apply vacuum on both sides. When you engage the pedal, you release the vacuum on the cabin side.

odonnell

When you say capping ports, you mean on the MBC?


Tonight I installed alternate pads on the front just to eliminate contaminated pads from the equation. When I started up the car, I got a soft pedal - very different from earlier. My suspicion is that all the testing today excited some air that must have been trapped in the master...it really did bleed a little too easily with my Motive. So tomorrow is more bleeding. I did a quick jaunt around the parking lot of my complex, and the brakes felt sort of ok when I really got the pedal toward the end of travel. Maybe it really was the master all along and I just didn't get all the air out despite bench bleeding it. I doubt the pads are even in the equation.

A new booster, along with the associated grommets and seals, is on order....it did fail the test after all. If it's not the culprit today, it could be next year.

Jfrahm

Maybe. It can be hard to get all the air out when the MC is changed. Overnight bleeding works well for me: pinning the pedal down for some hours with a scrap of wood, then bleeding one corner, then pinning the pedal down again, etc. etc. You may have to deal with the brake lights for this. I usually leave it for several hours for each position.

I think it helps because air bubbles get really small and over time can collect together, also you get really strong movement of the slug of brake fluid they may be living in. If they do not actually rise up to the top, they can get dislodged from the annoying hiding place.

Hard braking can also get bubbles to migrate to better spots for later bleeding also, for me this is more of an ABS bleeding issue though.

mel_t_vin

In my experience, a faulty MC [bad internal orings] usually manifests itself with the pedal going to the floor under braking. Your situation does not sound like bad a MC, to me.

Re: the brake booster...don't these 944 boosters [maybe others?] have a sort of 1-2 step function? Meaning, moderate pressure under light braking [like around town], but under heavy braking [like threshold braking from 100mph], an internal "poppet" valve is supposed to change state to greatly increase vacuum assist? Faulty "poppet" valve, maybe?

Does anyone have any additional information about the internal workings of our boosters?

odonnell

Well, the new booster is getting installed tonight, I removed the old one last night. Way easier than I thought. If it turns out that the old one was indeed bad (it is technically leaking but not dramatically), I'm not opposed to cutting it open to investigate the failure.

odonnell

Update for posterity / future troubleshooters

The issue ended up being the brake booster. After I installed it the other week and bled the system, it braked razor sharp. Passed inspection like nobody's business.

Last night I tore down the old '83-stamped Girling master cylinder, just out of curiosity. All the seals visually check out (I know that's not really scientific). The front and rear bores are mint, with a few areas where you can see evidence of seal travel. More of a surface effect than actual wear. Debating if doing a teardown of the brake booster itself would be worthwhile/fun. I suspect the issue isn't a blatant tear in the diaphragm, but a leak in the seals at the base.

V2Rocket

"yup, those look like seals alright."

StoogeMoe

Show pictures of that thing please. I've never seen what is in one of those mysterious canisters.