924Board.org

keithiopian · Posted: Tue May 30, 2006 6:42 am Post subject: master cylinder o-ring

i noticed my brake pedal going soft and brake fluid levels go down, and after searching around i found the leak is in between the master cylinder and the booster. according to my handy dandy haynes guide, there's an o-ring between them to seal the connection. problem is, i can't seem to find anywhere the size of the o-ring, as both the M.C. and booster are both in fine condition and only the o-ring needs replacing. my haynes says the master cylinder diameter is .813 in, but i doubt that's the size of the o-ring. any help on this one?!

thanks in advance,

keith

Ozzie · Posted: Tue May 30, 2006 6:57 am Post subject:

Look in the PET catalogue and you'll find a repair kit in the breakdown.
New springs and seals etc couldn't hurt.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

There is no such thing as everything is fine, except for this leak.

A leak is a leak, and requires complete rebuilding of the master cylinder, if you are not going to replace it. For the main seal (last seal to leak), there already has to be considerable internal leakage, which indicates that replacement or complete resealing is the correct action.

Paul · Posted: Tue May 30, 2006 10:14 am Post subject:

And some of us have found a large quantity of brake fluid in the booster....
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Smoothie · Posted: Tue May 30, 2006 11:18 am Post subject:

Yep (said the ol' timer) the real leak is in the master cylinders' seals. If you only replace the o-ring between mc and booster, the leak will continue unseen into the booster.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

keithiopian · Posted: Wed May 31, 2006 1:14 am Post subject:

so! looks like i'm in deeper than i thought. would you guys suggest the rebulit kit or is it worth it to buy a new master? also, what's the course of action to take if there is indeed fluid in the booster? thanks!

gohim · Joined: 02 Nov 2002 Posts: 4459 Location: Rialto, CA

Sometime ago, someone (Lizard?) noted that rebuild kits are/were available from NAPA for around $40, if I remember correctly.

While I did sucessfully rebuild master cylinders on several occasions in my younger years, I would look for a new one. I have been sucessful at finding new master cylinders at auto parts swapmeets for less than $50.

keithiopian · Posted: Tue Jun 27, 2006 12:42 pm Post subject: help!

so i found a new master cylinder, took the old one off put the new one on, no problems ('cept for the expected fluid in the booster, which i summarily cleaned out). Now i go to bleed the brakes. rear right: no problem! hey this is gonna be a breeze. rear left: nothin. unbolted the brake line, fluid comes out of there okay, but nothing out the bleeder valve. okay so somethin's wrong with my drum, right? i move on up to front right and nothing at all out of the bleeder valve. are ALL my brakes stopped up, or do you think this is indicitive of something else wrong? damn you gremlins! damn you personal mechanical incompetence!

Paul · Posted: Tue Jun 27, 2006 12:56 pm Post subject:

Did you bench bleed the master cylinder before attaching the brake lines?

What year is your car? It might have the diagonal system and the piston that provides fluid to one of the circuits is "sucking air."
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

keithiopian · Posted: Tue Jun 27, 2006 12:59 pm Post subject:

define "bench bleed." i ran fluid through it and pressed the plunger quite a few times to get the fluid though it, but if there's a more detailed bleeding technique for it, no i didn't (again, mechanical incompetence). the car is a 78, so not sure if it has a diagonal system or not. thanks!

Paul · Posted: Tue Jun 27, 2006 1:09 pm Post subject:

Now that you have it installed, just grab a towel and place it under the MC and one a time, crack and bleed each of the lines at the MC. Also make sure both sides of the reservoir have fluid in them.

Once done bleed all 4 wheels again.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

keithiopian · Posted: Tue Jun 27, 2006 1:14 pm Post subject:

great, thanks for the tip! i'll try it out asap.

Benji0301 · Joined: 18 Sep 2005 Posts: 66 Location: UK

I had a similar problem on my master cylinder & servo, unfortunately they don't sell master cylinder reapir kits for ATE in the UK due to apparently "possible liable suits from customers who fit their kits wrong then claim against ATE when their brakes fail & they crash" seems like BS to me and a way of getting you to buy the whole MC. I did manage to get a new MC from a friend in Germany in the end but still had the problem of the fluid in the servo. I took a bit more drastic action and decided to strip & rebuild it. If you want to do this follow these simple directions

1. You have to fit it in a vice and using an old chisel or screwdriver lift all the punched down tabs from around the edge of the servo.
2. Mark a line on both pieces of the servo so when you refit together it is in line (I had too take apart & rebuild so learn from my mistake).
3. Make sure you keep it in the vice (loosly) this is only to stop it springing open when you have released all the tabs. T
4. The spring will release with a bit of force so make sure you are prepared for this.
5. You will find inside a largespring, a large rubber bellows & a large metal plate. I cleaned up loads of dried up old grease & brake fluid using washing up liquid & water.
6. I gave all parts a thin coat of light oil & refitted the together making sure the marks lined up.
7. Use a large punch or engineers screw driver to punch down the tabs again.
8. I then gave it a coat of stain black paint for protecstion & fitted to the car.

It works a lot better now than it did before. Sorry if the instructions seem a bit disjointed but I am used to getting advice on the forum & not giving any.

Paul · Posted: Fri Jun 30, 2006 10:10 am Post subject:

Good work, mate.

That's the way it's done!

I suggest you copy and paste this into a new post in the "how to" section.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.