Rear Calipers dragging... BUT....

[CidTito]

Hi

So one of my several recent 69 911 projects have brought me an issue I want to sort on Tuesday. A little while back I had did some work on the pedal cluster and all in and working fine.

What I noticed after that work, was my brake pedal was seemingly a heck a lot higher (closer to me) than it used to be. I was surprise something changed..

I took off the floor board and using a 17mm wrench turned the bolt to bring the brake pedal down closer to front of the car. After putting it all back together, I had noticed that if and when I used my brakes a few dozen time or so (getting them warmed up), all sudden my stock rear calipers would start to slightly cause a drag on the rear wheels. This would not go away until the calipers became stone cold. There was no affect on the front calipers.

So.. off came the brake pads, and using a large screwdriver, massage the calipers back in a way to loosen the whole thing up and it freed itself up. The problem went away.

The brake pedal felt right, but I was curious to see what would happen if I turned it almost back to its original spot. But, when I did that, the calipers would return to clamping itself. Plus, now I've set the brake pedal too high.

QUESTION:

1. Will adjusting this 17mm bolt on the master cylinder push rod affect this caliper causing drag?
2. Is there something missing.. Bleeding?

The car has braided steel lines on it.

Brake Fluids look good, plus there is super slow drip coming from the master cylinder.

[Flieger]

Dripping master cylinder is bad. Replace that.

You don't want the pushrod too long or else the brakes won't fully release. Making the pushrod shorter should lower the pedal.

[CidTito]

Thanks - ok -

I'll shorten the pushrod, what happens to the calipers?

[Flieger]

I think you had the rod too long to begin with. The slight dragging of the calipers caused the brake fluid to boil and because the rod was too long, the port going from the master cylinder to the reservoir was closed, thus allowing pressure to build up and cause the rear pistons to extend even more and lock up. Shortening the rod on its own would not retract the pistons enough, but the screwdriver would. When you made the rod longer the problem returned, correct? I would say make the rod shorter until there is a little bit of clearance between the end of the rod before it engages the master cylinder. There must be a spec for this somewhere. You will then need to spread the brake pads again but the calipers should behave normally afterwards.

[CidTito]

I read a spec that essentially matches the height of the brake pedal with the height of clutch pedal. The way mine is now, is that its over the height it needs to be.

Ill do exactly as you suggested, shortening the engage spot on the rod..and then spreading the pads one more time. I can be assured to have more free play that wasn't ther for a pretty long time. I am sure this has to be problem and the reasoning of why it behaved that way.

keep you posted.

.t

[Flieger]

Just make sure that the rod has a small amount of clearance before engaging the master cylinder.

[CidTito]

Ahhh... worked!!!

I found at spec that states in resting position, the height of the clutch pedal and height of the brake pedal should be equal. So, just today I shortened that push rod to a new point that now they both line up. I'll be driving around a bit with its new position and see if I like it.

Very helpful

.t

[Mr9146]

The first adjustment at the pedals is the push rod to master cylinder clearance like Max pointed out. Then you adjust the clutch pedal height to match the brake pedal.

There is no spec for clutch pedal or brake pedal "height", only the amount of play between push rod and master cylinder. Everything else tailors to fit driver preference/comfort. However, if all factory adjustment specs are followed initially, then the pedals should come out pretty even from Jump Street.

[motogman]

Just make sure that the rod has a small amount of clearance before engaging the master cylinder. 

Max made a couple of posts and I think this one perhaps points more at the issue.

Within the master cylinder, when the pedal is released, the hydraulics in the brake system are open to the brake fluid reservoir so that there is no pressure in the system. When you first start to depress the pedal/master cylinder, a port or valve within the master cylinder closes, there by isolating the brake system from the reservoir so that pressure can be built. When you release the pedal back to the rest position the valve/port should reopen to relieve pressure.

If the input rod to pedal connection is not properly set (and it is too long), then there is the possibility that when you release the pedal, the valve/port will not fully open and you can have some pressure in the system which will cause drag. Due to the different tolerance stack conditions, it is quite logical that this could happen on one circuit and not the other.

This sounds to me, the mostly likely cause of your issue. Other things that could be a factor... I think (but don't know much about it) that some P cars had a proportioning valve that limits the pressure to the rear - these can sometimes go bad. Even though you pushed the pistons on the caliper back in there could be retraction issues due to corrosion on the piston or other issues or there could be pad slide issues either on the pins or abutments causing the drag. Pulling the pads and cleaning inside the calipers with a little wire brush and an air gun can remendy the pad slide issue. There could be some issue with the m/c causing more friction so it does not return all the way and if this were the case it could be related to the slight leak.

I do suspect it is the pedal rod adjustment putting a little excess force on the m/c and it not releasing fully but I also agree with Max that if it is leaking a little, the best thing to do is replace the m/c (then do all of the proper adjustments).

[Flieger]

The proportioning valves appeared on the 3.2 Carreras.