Brake Diagnosis

[Porsche_monkey]

Replaced most brake tubes and all hoses. I have pressure bled my brakes (74 911 - stock brakes) and they still feel a little soft. No apparent leaks anywhere.

I also lose some braking as I sit at a stop with the pedal depressed. Brake pedal seems to creep down very slowly, a quick pump will rebuild the braking pressure.

I'm thinking my master cyclinder needs to be rebuilt.

[Rot 911]

Was everything working ok prior to the replacement of the hoses? If so I think you still have air trapped in the system.

[Porsche_monkey]

Sorry, never drove it before I did this work.

[JeremyD]

Do you have any leaks - any visable fluid? Check around your MC -- pedal cluster... All lines properly torqued?

Speedbleeders or regular?

I just did the same - except I changed out the master cylinder, all soft lines and hardlines, 4 new calipers including two I rebuilt. It took me a gallon - yes a gallon of brake fluid to get the pedal hard. Tap on the caliper with a hammer while you are bleeding it. you would not believe all the places that air pockets can get.

[Noel]

A pedal that creeps down when at a stop light usually indicates a bad master cyclinder ("MC"). Check for leaking brake fluid above the drivers side foot well. This is how I noticed mine was bad. However, leaking fluid is not always necessary to have a bad MC.

Good luck.

[Porsche_monkey]

Originally posted by Noel
A pedal that creeps down when at a stop light usually indicates a bad master cyclinder ("MC"). Check for leaking brake fluid above the drivers side foot well. This is how I noticed mine was bad. However, leaking fluid is not always necessary to have a bad MC.

I was thinking soft pedal meant air (providing I don't have a leak), but a creeping pedal meant the master cylinder was bypassing.

[JeremyD]

I've had both - usually depends on the volume of air.

[Porsche_monkey]

I do not have speed bleeders. I ahve stock Porsche bleeders. I modified my brake fluid cap so I can connect 5 psi compressed air to pressure bleed it.

I do not believe there are any leaks.

[JeremyD]

First thing I would do is check for fluid around your MC. If no fluid then I would again do a good bleeding. If the hard pedal doesn't return - or you get the same creep I would change out your MC. That's what I would do.

[911mot]

Clamp off the flexible pipes,.If the pedal goes hard its a probably a weeping caliper or join. If it continues to sink almost definately MC.
Very common for a MC to go bad after a brake bleed. The rubber seal on the piston gets damaged during the bleed process because it is asked to travel much furhter down the cylinder than during normal everyday use. This area of the cylinder is, on a older cylinder, usually coated with a build up of grime wich tears at the rubber seal.
A top tip on older cars is to bleed the brakes in the usual manual way but only depress the pedal 1/2 way. It takes longer but saves damaging the seal.

[JeremyD]

Good point 911mot

[911mot]

And please dont use pressure bleeders. They are a waste of money IMHO. They are not worth potential paint damage.

If you want to beed brakes on your own here's how:

You will need a large clear jar (milkbottle/pint glass/jamjar etc) a length of rubber hose and some fresh fluid.
Pour a little clean fluid into the jar.
Starting with the caliper furthest away from the MC, attatch rubber pipe to caliper bleed nipple (ensure tight fit) and place unopened end of pipe BELOW surface level of fluid in jar.
Crack bleed nipple very slightly
Pump brake pedal SLOWLY and only halfway approx 10times (or until bubbles stop appearing in the jar if you can see it)
Release pedal and tighten bleed nipple.
Top up MC fluid level and change fluid in bottle for new.
Proceed to next caliper and repeat until done.
As long as the end of the rubber pipe stays sumberged in the fluid in the bottle, you will not draw air into the system with the pedal released.

[Porsche_monkey]

Originally posted by 911mot
Very common for a MC to go bad after a brake bleed. The rubber seal on the piston gets damaged during the bleed process because it is asked to travel much furhter down the cylinder than during normal everyday use. This area of the cylinder is, on a older cylinder, usually coated with a build up of grime wich tears at the rubber seal.

That's why I pressurized the reservoir. You don't have to risk damaging the M/C.

[Noel]

IMO, the Motive pressure bleeder is the ONLY way to go. I've flushed my brakes about 12 times with it. A true one man bleed job.

[makaio]

Master cylinder is my vote. I replaced or rebuilt every part of my SC's brake components except the master cylinder. Bled the brakes multiple times and still had soft brakes, untill I pulled the master cylinder off and discovered it had been leaking ever so slightly. Chances are yours is the original, and needing to be replaced.

As for the Motive Power Bleeder, it's the only way to bleed brakes by yourself IMO. Just make sure to clamp off the overflow tube, and don't pressurize more than 15 PSI.

[Porsche_monkey]

Sounds like something to do this winter. Thanks for your input everyone.