Brakes seem to be constantly on. 1984 911

[geoff_k]

Hi

Noticed this a few months ago.

What happens is the brakes seem to stay on. There is an obvious drag.
At the time I bled my brakes and it was fine for a few weeks. I do not drive it a lot and that means it might be two weeks before it get a good run and gives me a good feeling (yes I should drive it more).

It happened last November and I went through and re bled the brakes and made sure I change any fluid. Brakes were fine afterwards.

Took it for a run yesterday (after two-three weeks not being driven) and the same thing. Can feel the drag. The pedal feels hard. I did note the pedal has not returned completely and if I just my heel to pull it that inch or less back then there is no drag.

I took it for a run and had to continually pull the pedal back with my heal to stop the drag.

The car is an 1984 911. Master Cylinder was changed out about two yrs ago.

[kltarga72]

Maybe a stuck wheel cylinder? Tires off the floor will all four wheels rotate with only a slight drag?

[911obgyn]

Check pedal bushings and springs

[chrismorse]

Originally Posted by kltarga72

Maybe a stuck wheel cylinder? Tires off the floor will all four wheels rotate with only a slight drag?

If only one wheel, may be bad rubber brake hose - change them all. More likely pedal bushings.
:-)chris

[jlex]

If you're having to pull the pedal back which then stops the drag, it must be a problem with the pedal mechanism itself... pull the carpet back and look at the pedal cluster to see what's going on. May be time for a rebuild.

[RWebb]

how old are the rubber hoses????

[gomezoneill]

Originally Posted by RWebb

how old are the rubber hoses????

^^^^^^^^^^^this^^^^^^^^^^^^^

[Walt Fricke]

Old rubber brake hoses at the wheels are a traditional suspect. It appears the lining can split and act like a sort of one way valve. But since the hose is multi-layer, it doesn't leak.

However, pulling back on the brake pedal with your foot wouldn't really cure this situation, and typically this one way valve thing you would expect to affect only one caliper, at least at first.

When the brake peal is fully retracted, the holes in the master cylinder, which connect the fluid reservoir to the internal piston chambers, are open. As a result, the brake fluid and lines are all at atmospheric pressure - and hence exert no force on the pistons in the calipers. When you step on the brake, the pistons move forward, closing off these holes, thus allowing the pedal to create the pressure you need. If the MC piston(s) don't retract fully when your foot comes off the brake, the pressure in the brake lines won't bleed off, at least not fully.

The long rod connecting the pedal to the MC bell crank was adjusted in length so that the piece coming out of the power brake canister has just a little bit of play where it contacts the outermost piston of the MC. This guarantees that the spring inside the MC can fully retract the pistons. However, since your system worked initially after the new MC was installed, this long rod is probably properly adjusted.

If, somehow, the vacuum isn't being entirely released, that could cause a drag. I wouldn't rate that as high on the suspect list, but you could disconnect the vacuum (probably a good idea to plug the hose so the engine isn't affected) and see if this affects the pedal return.

I agree with those who suspect the pedal cluster. If the bushings in the pedal cluster have become gunked up and stiff, that fits your symptoms, even though the pedal return spring is fairly stout. The bushings do wear over time, though usually that manifests itself in sloppiness and sometimes issues with clutch release. A quick test might be to spray penetrating oil into the brake pedal bushings while wiggling the pedal back and forth. You need to remove the floor board to do that, and welcome to the Houdini pretzel club, but lots of us have worked on pedal clusters.

Whether or not this band aid helps, a cluster rebuild is in your future. Pelican doubtless sells a bushing replacement set, either nylon as the original, or bronze which most use for replacement. Plenty of posts on how to deal with the pedal cluster. Our host Wayne's book 101 Projects for your 911 at chapter 40 has an explanation. If there is a problem with the brake bushings, chances are the clutch bushings there are also worn.

[Bill Douglas]

You need to put it on a hoist and see which wheels are locking up. Then work from there.

If it's the back wheels it's probably your parking (emergency) brake in need of adjusting or some work.

[thor66]

replace all the rubber hoses; inspect each wheel cylinder; pad thickness; rotor thickness & runout using a dial indicator; check parking brake 'balance' and total # clicks

put in new fluid and test - if good replace the fluid every 2 or 3 years and sin no more

if the brakes still stick, then you will do post #9 and go from there

[Mahler9th]

I had this happen with a 951 way back when. I was supposed to drive in a weekend track event at Laguna Seca and the symptoms occurred that morning on the way to work that Friday morning, and then on the way down to Salinas after work I had to divert and go home instead. I missed the event.

What would happen is that I could drive a while and the brakes would start dragging more and more until it was clear that I needed to stop. Then I'd sit on the side of the road for maybe 20 minutes and start the cycle all over again. While I sat there I could smell the friction material. I was lucky that I did not ruin my expensive 965 rotors.

This was a highly-maintained street/track 951.

The culprit in my case was a leaky seal between the brake fluid reservoir and the booster. I discovered it on the next day (Saturday) right after lunch... I had checked a bunch of things and then gone for a drive. After a few blocks the problem re-occurred.

I stopped on the side of the road, popped the hood and used a small flat blade screwdriver to gently pry the check valve out of the booster. There was a whoosh and voila, the issue went away and I was able to drive far enough before it cropped up again to get home. When I got home I removed the master cylinder and used my Mityvac tool to see if I could suck fluid out of the booster. I got a few ounces. Problem identified. I quickly washed up and jumped into my other 951 and made it to Partsheaven just before they closed (3 pm). I bought a good used booster and installed it the next day.

I made sure to use a fresh master cylinder seal.

Never happened again.

Good luck.

[frankc]

Originally Posted by Walt Fricke

If the MC piston(s) don't retract fully when your foot comes off the brake, the pressure in the brake lines won't bleed off, at least not fully.

^This. If the pressure cannot be released due to the pedal not retracting all the way, then the brake pads will drag, which will cause the fluid to warm up and expand, which puts more pressure on the brakes, which causes the fluid to get hotter and expand more.. You see where this is going.

[Joe Bob]

If it is just one brake dragging a hand held pyrometer will tell you after a drive. Shoot a temp reading on all four of the rotors. The hot one is your problem. If it pulls one way or the other try that side first.

[hitch911]

Identified original brake hoses on my 85 carrera. Seems like a good measure step...right?

[Walt Fricke]

Joe Bob gave you good diagnostic ideas:
a) At speed, when you step on the brakes, does the car pull?
b) Use an infra red heat gun (these are inexpensive, and are what places point at your forehead these days to see if you have a fever) on your brake rotors after driving around.

If it is just drag on one rotor, a) may not really happen. But b) will show up pretty clearly. If neither do, that might point to a problem with the MC, because if all the brakes drag, the MC is really the only central point. It isn't likely that more than one rubber brake hose is acting up, much less that all are. If the problem is isolated to one rotor, that would suggest replacing that brake line, and it only makes sense at that point to replace them all. I don't think it is guaranteed that original brake lines will necessarily fail. On the other hand, replacing them isn't a waste, even if it turns out not to cure the problem.

[911 SLANT]

I had this same problem. It was the shaft pin on top clip came off and pin was rubbing against body. The rod that goes up to booster. You have a 1984 car, so have the same.

[911pcars]

Diagnose first, then repair as needed. Buying more than you need is an exercise in shopping and/or self-gratification. YMMV.

Sherwood

[hcoles]

Originally Posted by thor66

replace all the rubber hoses; inspect each wheel cylinder; pad thickness; rotor thickness & runout using a dial indicator; check parking brake 'balance' and total # clicks

put in new fluid and test - if good replace the fluid every 2 or 3 years and sin no more

if the brakes still stick, then you will do post #9 and go from there

I have one or two front brakes dragging a bit. I have all new hoses from our host. What are the tricks to remove the old "rubber" hose parts and install new ones without having all sorts of fluid coming out? Thanks.

[frankc]

Originally Posted by hcoles

What are the tricks to remove the old "rubber" hose parts and install new ones without having all sorts of fluid coming out? Thanks.

Place a block of wood between the brake pedal and the seat to keep the brake applied lightly. This will cause the master cylinder to seal off the brake circuits from the reservoir. And don't forget to remove the wood block before bleeding the new lines .

[hcoles]

Originally Posted by frankc

Place a block of wood between the brake pedal and the seat to keep the brake applied lightly. This will cause the master cylinder to seal off the brake circuits from the reservoir. And don't forget to remove the wood block before bleeding the new lines .

Why didn't I think of that? Thanks very much.