Spongy 911 SC Brakes Mystery

[TomFalkiner111]

Hi guys. I wonder if anybody can help with a mysterious brake problem. In the beginning of last year my brakes were perfect. Nice firm pedal, the car would stop on a dime. Then I went and put Castrol SRF brake fluid into it. Big mistake. In just a matter of days my brakes got softer and softer. So I drained the SRF and replaced it with DOT 4. Too late. The damage was done. The seals in my master cylinder had been corroded.

So I ordered in a brand new OEM master cylinder, upgraded to stainless steel brake lines, serviced all the calipers and installed new OEM brake discs and EBC Green Stuff pads. When everything was bolted into place I filled up with DOT 4 and bled all the air out of the system. I expected the car to return to its old braking self but it hasn't.

The brakes are still soft. Especially at lower speeds in traffic. If I apply a slow and constant pressure on the brake pedal it will eventually go all the way to the floor and the car will still keep on creeping forward. At much higher speeds with a sudden, jabbing force the brakes feel a bit better but nowhere near as good as they once were before I put in the SRF.

I don't know what to do! Does anybody know what might be causing the problem? Could it possibly be vacuum related? Any advice would be most appreciated!

[SoCalSK8r]

SRF is drop in compatible with DOT4 - why do you believe you had a corrosion issue? How long did you leave it in the system before you had pressure issues?

It still sounds to me like you have air in the system, did you try tapping the calipers with a rubber mallet to get all the air out?


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[pors1968]

You should state model year .

[Jcslocum]

The drifting to the floor is indicative of a bad master cylinder. The fluid is slowly bleeding by the seals under light pressure. The hard push makes a better seal to the bore because of the higher pressure pushes the seals out to the wall better.

The booster is just that, it helps with lowering effort but has nothing to dd with the hydraulics.

If there is no fluid coming out, I think you need another master.

[TomFalkiner111]

It's 1980 911 SC

[TomFalkiner111]

Hi guys. Thanks for all the replies.

The SRF ate the seals inside my master cylinder. This was the original master cylinder that the car came with from the factory (34 years old). I had this master cylinder reconditioned with new seals and changed the brake fluid back to a more pedestrian DOT 4 brake fluid. Things only improved slightly. After this I ordered a brand new ATE master cylinder from Pelican Parts and fitted it to the car along with new pads, discs and stainless steel brake lines. I also had the calipers serviced.

After all of this the pedal still feels mushy. At low speeds, if apply a gradual force, the pedal will travel straight to the floor and car will slowly creep to a stop. Very annoying as I've basically overhauled the entire brake system. What could be causing this???

[NoLift911]

Air in the system. Not sure if you bench bled the M/C or how many passes you made on bleeding. Given an overhaul and not sure if you are using a mightyvac or not but you need many many passes and a final couple person in the car method for a good bleed on an overhauled system.

Yes I know you had the issue before the overhaul but now - its air.

[TomFalkiner111]

I'm also not sure. My mechanic did the install. So do you just suggest a long session of brake bleeding?

[Bob Kontak]

I'll go with air.

Is there a way to bleed the master in place? Man, access looks tough on my 81. Would one way bleeders work on it (aside from the mess in the trunk and paint damage risk)?

[DRACO A5OG]

Do you have a proportioning valve?

When you serviced the calipers, how did the pistons look, were they pitted at all?

I could never get my brakes hard enough. Then I found two issues, a leak at the Proportioning valve and 3 out of 4 pistons were pitted, swapped Proportioning Valve and all rear pistons, rock solid brakes now.

+1, try bleeding couple more times, remember to place a block behind the pedal if doing the 2 man team bleed, you do not want to damage your brand new MC.

Jim

[scarceller]

Are you sure you don't have excessive wheel bearing play? Check all 4 tires be sure that with car up on lift and wheels off the ground none of the wheels have any lateral play.

[uwanna]

Here's a thread addressing problems with the new ATE MCs that are now available from PP. John Walker started this thread complaining about the exact symptoms you are experiencing. Not very promising news!

ATE master cylinder issue?

[midnight911]

i had an experience with power bleeder having trouble bleeding air out. Use conventional up...down...up...down...stigma, which worked for me as it seems to push the air out with greater force.
also, other times, when the pads were not yet worn and seated to the rotor surface, i had some mushy pedal. few high speed braking cured this problem for me.
check all the hard lines and connections for possible leak somewhere else.

[reddogmotrsprts]

If the pedal is firm, but keeps slowly traveling towards the floor, then the problem sounds like MC seals. If it's just soft all the way down, then it sounds like air in the lines. It often takes a couple bleeds to remove all air from a new master. You should be able to diagnose the master by watching the movement of the fluid while bleeding the system. Remember to check the front and rear brakes separately.

[Cooper911SC]

Here's another thought. You mention having the calipers "serviced". If the seals were replaced, they take time to bed in. Even with a coating of brake fluid or sil-glide on rebuild the pedal will often feel soft on initial push.

This is caused by the square section piston seal slightly grabbing and pulling back on the Pistons. Very similar to Knock Back of the pads on a track car.

No amount of bleeding will fix it. Just have to put a few hundred miles on the car and get new seals broken in.

Just another thought.

Cooper

[Bill Douglas]

A friend of mine swapped in new calipers and could NOT bleed them. Always spongy. He gave up and took the car to a mechanic who noticed the calipers were on upside down. No wonder the air couldn't be bled out. That was on a '79SC.

All the best, Bill '79SC.

[Quicksilver]

Originally Posted by Cooper911SC

Here's another thought. You mention having the calipers "serviced". If the seals were replaced, they take time to bed in. Even with a coating of brake fluid or sil-glide on rebuild the pedal will often feel soft on initial push.

This is caused by the square section piston seal slightly grabbing and pulling back on the Pistons. Very similar to Knock Back of the pads on a track car.

No amount of bleeding will fix it. Just have to put a few hundred miles on the car and get new seals broken in.

Just another thought.

Cooper

If it is serviced with non ATE seals it may never work correctly. The cheap seals have WAY too much seal retraction. I saw a fresh rebuild with over 2500 miles on it never get over the seal problem.

Insist on ATW in the blue box.

[Tom '74 911]

Originally Posted by Cooper911SC

Here's another thought. You mention having the calipers "serviced". If the seals were replaced, they take time to bed in. Even with a coating of brake fluid or sil-glide on rebuild the pedal will often feel soft on initial push.

This is caused by the square section piston seal slightly grabbing and pulling back on the Pistons. Very similar to Knock Back of the pads on a track car.

No amount of bleeding will fix it. Just have to put a few hundred miles on the car and get new seals broken in.

Just another thought.

Cooper

If your calipers were rebuilt with new seals, this could be the culprit. A soft pedal is the symptom and for me, cleared up after a few 100 miles.

Air in the lines is another obvious, easy thing to chase. I usually bleed my brakes a few times with a few drives in between to get all the little bubbles out for good.

Neither of these will necessarily cause a "pedal to the floor" symptom though.

Tom

[Jdub]

When we hear hoofbeats we don't think Zebra. The new fluid has nothing to do with the MC - it was your bleeding technique. And if there was air in there before, there are now many other issues that you now need to factor in since you've essentially done a total replace.

Remember pads must, over time, bed to rotors, so that is slop that will eventually leave the system. There is natural stiction if you did replace the caliper body seals, so there is that. Air, of course, that only leaves over time.

Plenty of great information via a Search. Take a look for material I posted on my recent rebuild and work from there to smarter folks than myself. You will have an awesome set of brakes soon enough so just hang tight!

[Tidybuoy]

When I replaced my MC with a brand new ATE, I had the same symptoms as you (pedal going to floor). After many power bleeds and manual bleeds, I ordered a 2nd new MC. The second MC worked so something was definitely wrong with the first.