odonnell

After testing my original '83 Girling booster with a Mityvac last month, I picked up on a small leak and ended up replacing it. So for science, I tore down the old one to see what's inside it, and if anything makes it special or unique compared to other brake boosters. Pardon my fumbling around the whole time, never taken one apart before.


There are no rebuild kits (to my knowledge) for the 944 booster - although other parts from similar era boosters are probably a direct fit...Girling and Teves/ATE pretty much have the entire German market. IMHO, not really worth the bother to rebuild, but maybe this will help people who want to know more.


Here's a clip of how the booster works, as far as the vacuum valving:


Link to a new booster for under $200 shipped

These parts are going in the trash unless someone wants a part or 2 from what I have left. Maybe I can do something with the diaphragm, like make a cool party hat.

mel_t_vin

Nice job, Michael.

Over the years, have heard several folks complain about an increase in pedal pressure required to slow the car down. Along with that, some folks have also mentioned an inability to lock up the front tires.

Reviewing your videos, suspect the former symptom could be manifested by a tear in the diaphragm, and the latter symptom by damage to that spring-loaded, shaft-mounted, tapered cup seal.

Thanks for tearing one of these apart.

odonnell

My major issue was high braking force required and poor performance. Master cylinder already replaced at that point, and everything from the flex hoses forward was in excellent condition, sort of just leaving me the booster by elimination.

My diaphragm looked ok - although I can't rule that out, as I have no way to test the assembly in equivalent operating conditions with it all being hacked up. It brakes awesome now though


Test data from before teardown:

- Mityvac used to put 15 inHg on the forward chamber via the check valve, which tested good. Dropped about 1 inHG every 3-5 mins, indicating either the diaphragm was leaking or the atmospheric air was leaking into the aft chamber via the shaft seal, which we know is open to the forward chamber when the pedal isn't pushed.

- With 15 inHg applied again, pushing the brake pedal shows ~5 inHg dropping, like a step change. That's from the diaphragm moving forward, and making the entire pressurized space contain a lower volume. Thus, pressure increases (err, becomes less vacuum-y). At the same time, the shaft seal is supposed to be sealing off the forward and aft chambers. When the pedal is released, the two chambers are once again connected. Atmospheric air that was in the aft chamber is now part of the test circuit, so you still read the same lower pressure even after the diaphragm is relaxed.

- Same test as above...apply 15 inHg of vacuum and hit the pedal. Then you hold it there. In my case, it immediately dropped ~5 inHg just as it did before. If the booster was working perfectly, you could hold the pedal down for a long time and still see ~10 inHG. The shaft seal should be allowing atmospheric air into the aft chamber, but also keeping it blocked off from the forward chamber...so no reason for pressure to drop at the forward chamber, where the reading is taken. This was not the case for my booster, it lost about 1 inHg every minute. Hence why I believe either the shaft seal failed, or there was a leak along the diaphragm seam.

mel_t_vin

By shaft seal, do you mean that spring-loaded, shaft-mounted, tapered cup seal?

So, after installing a new booster...night and day? Your high pressure/poor performance issue has been resolved?

odonnell

Yes, the seal on the spring cup. It wasn't blatantly torn but who knows how well it was doing its job.

Braking is DEFINITELY night and day. It was simply unsafe before. It was a sudden thing - I remember driving to work one day and having to really push the pedal to slow down. I replaced the master and it felt the same (no regrets though, based on the brake booster front cover, it had a slow leak). The booster did the trick, this is the best it has ever braked under my ownership. It's also rocking some Volvo 4-pots up front

When replacing it, one must check pedal free play. Going from a Girling MBC and booster to the ATE MBC and booster required a few mm of adjustment. The pedal would engage too low. Easy fix, and you can interchange Girling and ATE parts too. ATE is just less expensive and the quality is still there.

mel_t_vin

Probably hard to ascertain if it was torn...though, based on your description of the issue, suspect it [poppet valve?] was the root cause of your issue.


Have heard the exact same thing [sudden onset] in several cases from different individuals.


Yup...remember your DIY write-up several months ago. Nice.

mel_t_vin

Out of curiosity, were you able to separate the MC from the BB, to remove it, without disconnecting the hard brake lines? Or was there not enough slack/flexibility in the hard lines?

odonnell

I removed them. In order for the booster to come out it has to slide forward considerably to clear the pushrod, I remember it hanging up on the brake lines even after they were disconnected. On the early car, I also had to remove one of those plastic clips that holds the fuel hardlines to the firewall. It's about the 9 o'clock position on the booster assembly.

jeffro951

Anyone ever used a girling in place of an Ate? I know the M/C is specific so I plan on using the girling M/C as well.

odonnell

You can mix and match. Just adjust the pushrod clevis (freeplay) to compensate. I ran a Girling booster with an ATE master cylinder for a few weeks before replacing the leaky booster. They're basically interchangeable if you adjust it.

jeffro951

Swapped a Girling booster for an Ate on my Cabby and the Girling is longer so the passenger side line on the M/C hits the intake manifold. I will be swapping the 16v engine for and 8v so it shouldn't be a problem then. I think the S2's and the turbos need the Ate booster for clearance issues.