jmartins

I have one question about brake bleeding, and once this is the first time I'm doing this in a Porsche, I like to know if some behavior is normal.

I'm using the old school way, first bleeding the master cylinder, and them each caliper. But after about 15 seconds, the pedal looses it's pressure, and after press the pedal for about 4-5 times, the pressure is back.

I think I have a bad master cylinder, but I'm not sure about the pressure regulator.

Any thoughts ?

Thanks

JL

fraggle

it sounds like a master cylinder to me, but considering how big a pita it is to get in and out, I'd wait for a second opinion!

Landseer

Doing a normal old school bleed on a 928 what you described will not happen.
Master.

NoVector

If it worked fine before and now it doesn’t, any possibility the reservoir fluid got too low or air was sucked back in from the nipple/hose while flushing? I.e., your friend lifted the brake pedal before the nipple was closed. The air wouldn’t compress like fluid and would take more compressions to build pressure. Personally, I'd reflush before buying a new MC. That's assuming it worked fine before... I also thought the MC was bled last--start with the caliper furthest from the MC and work your way towards it (?)

jpitman2

Last time I flushed my fluid, I used a MityVac to pressurize the reservoir (careful with the pressure!), and opened each bleeder until new fluid come through, pedal was same as before afterwards. Only issue was that the whole reservoir came off when I went to remove the connector from it - new grommets required .
I have had good luck with pedal after things like reservoir change, and brake light switches, by minimizing fluid loss, and allowing trapped air to migrate out by itself overnight.
jp 83 Euro S AT 54k

dr bob

If the brakes were OK before the bleeding and you have a soft but 'pumpable' pedal after, it's almost undoubtedly due to air in the system.

There are some dangers to doing the bleeding with the classic manual method, where a helper pushes on the pedal as you operate the bleeders at the calipers. This is especially important in a system that's been undermaintained sometime in its past. The master cylinder consists of a pair of metal pistons, actuated by the pedal via the booster. Each piston has seals on it, dished so that increasing pressure in the system will push the edge of the seal tigher against the bore of the cylinder. The bore is originally smooth end to end. As the car ages, especially if moisture accumulates in teh fluid, a bit of corrosion may occur in the bore. Any stray bits of corroded metal get swept to the forward end of the bore. Over time, it's that corrosion that causes the seal to be damaged. Pushing the seal into the collection of debris also risks damage, as one would do when manually bleeding the system.

I've been religious about flushing the system on my cars since I bought it, using a home-made version of the Motiv power bleeder. In spite of that annual use, I ended up with a rather low pedal a few years ago. I'd bleed it time and time again trying to get all the air out, but somewhere there was a bit of air captive and it wasn't coming out with the power bleeder or with manual bleeding. In the end, I used a combination of the vacuum bleeding technique with the hand vacuum pump, a container with brake fluid and some hose, and the power bleeder. I performed a "bench bleed" on the master cylinder, then disconnected, cleaned, flushed and vacuum-refilled each section of tubing starting at the master cylinder. I did a vacuum bleed on the ABS unit, then worked my way out to each of the wheels, pulling and/or pushing fluid uphill through each section of tubing. The combination of all these steps has the pedal back up where it's supposed to be, and as firm as it's ever been. I'd been following Nicole's adventures of a few years prior, during which time she chose not to drive the car at all due to the low/soggy brake pedal.

Anyway, if you've read this far, you know that manual bleeding especially in an old car is generally not a good idea, as it can lead to more problems than it solves. Pressure bleeding is better, since it also flushes the system some, and there's minimum risk of accidentally allowing the reservoir to go dry.

On when to bleed the MC-- The bleeder nipple on the MC only bleeds the front of the forward chamber. Bleeding there is most important when the installed master cylinder sits nose high, such that air can accumulate in the end. The hydraulic lines attach to the master cylinder body down lower, so air will not be pushed out of the front of the bore. The MC bleed nipple easily gravity-bleeds (no pumping of the pedal needed or desired) air from that space. That should always be done -before- bleeding at the wheels.

Ducman82

power bleeder.. only way to go!

Landseer

New master.
Car sat 15 years.

Same for any others being rescued with crud in the bore.

On the pre S4, rebuild of all four calipers is also a very sensible move.
New seals, new sliders. Runs about $60.

brealytrent

My father and I did it the 'old fashioned way' with zero hiccups. And I had installed a new hard brake line to the ABS, brake hoses all around, and rebuilt calipers.
Just lucky?

FiveElements

I had similar issues. My fix was replacing the rubber gromits under the reservoir tank as they were worn and kept letting air pass into the line. Then u must be sure to bleed every capliper piston. (4) per wheel.