Brake seizure problem

[Bambo Jr.]

I am looking for a solution to a problem that my 1963 220SEb Vin# 111.023 120 39550 has developed. This 111 model has front disc brakes and a non-tandem master brake cylinder. Even though my factory Mercedes maintenance servicer manual covers 1959-67 models, it discribes only tandem master cylinders and not non-tandem. Apparently my master cylinder's internal design does not require a special check valve to relieve pressure on the front disc but does have a primary check valve to trap fluid going to the rear drum brakes. This design is quite different when compared to the illustrated later tandem master cylinder, which includes a "special" check valve on the line to the front disc brakes, which releases pressure on the discs once the brake application is removed. I recently had the Master Cylinder and the remote ATE50/26 Power brake accumulator/slave rebuilt. Now I am experiencing occasional brake lock up! The only clues that I can provide is that once the brakes lock up: 1. I can free them by opening a bleed valve on the boost accumulator slave cylinder. 2. While it is locked up, if I unscrew the hydraulic line going from the master to the booster, fluid slowly dribbles out, but if I unscrew the line from the booster returning to the master cylinder it spurts out under pressure. On the surface I would assume that my problem is not with the rebuilt master cylinder but somewhere in the rebuilt booster or slave cylinder that is maintaining pressure in that booster to master return line. If that is the case, where in the booster/slave is my problem? Does anyone have any thoughts?

[Tom Hanson]

Replace your front brake flex hoses. These collapse internally allowing the pressure to go into the caliper, but not return. Your bleeding to release the pressure describes the exact symptom.

[Bambo Jr.]

Tom, Thanks for the input but I'm afraid that is not the problem. As part of the brake overhaul, I had replaced all the brake flex hoses with teflon coated flexible stainless steel lines, which I was under the impression was the ultimate upgrade and would prevent the problem you discribed.

[ja17]

Hello Greg,
Make sure you have the correct "free play" when the brake pedal is released, or fluid may not be able to return.

Joe Alexander
Blacklick, Ohio

[Bambo Jr.]

Joe,
Thanks for the input. Yes, the pedal freeplay is correct. Today I isolated/ bypassed the booster and the brakes are responding normally (no seizure), although of course without boost, so I am assuming that the rebuilt booster or its slave are at fault and will return it to the rebuilder under warrantee.

[joeym7]

Hi before you reyurn part the banjo fitting is connected to a duel check and release valve make sure it is working and vac supply is correct.joeymone other thing on your 111 you may also have a second slave cylinder over rear axle for the drum brakes you may want to check its presure and operation to .

[Bambo Jr.]

Joeym7, I looked around and I am not sure what banjo fitting you are referring to. Could you discribe it and its location and its purpose? Also couldn't see/locate what might be considered a slave cylinder in the rear axle area. As I mentioned, after I isolated and removed the booster assembly ( I ran a new short line from the "out" to "in" ports on the master cylinder that used to be the two to and from lines servicing the booster), my brakes, both front and rear, are working normally but requiring increased pedal pressure because of the lack of boost assist. Since I now have brakes, I have concluded that the removed power boost/slave cylinder unit is the culprit.Thanks for the input. See non-tandem master cylinder model at the lower right corner of attached picture,

[joeym7]

Hi the banjo fitting is on top it connected to the threaded valve on booster.joeym

[Napoleon Hodges]

FYI, it's the shiney brass piece on the valve on your booster. See your jpg of the booster.

[Bambo Jr.]

I had returned the overhauled brake powerboost/slave assembly to the rebuilder. They called today to inform me that they disassembled the unit and everything was in order. To verify proper function they tested it thoroughly on their specialized test stand. The rebuilder did say that the differential between the boost and the master is approximately 50 to 1 ratio. It is now being returned. I appreciated all the Forum input, but I am now stymied with regard to my discribed brake lock up situation. I've overhauled the master, the booster & slave, the front disc assembly and changed all the hoses to flexible stainless. The only suggestion not replaced is Joeym's "dual check and release valve" under the banjo fitting on the booster. I was under the impression that its only function was to act as a one way check valve to keep vacuum from leaking out. I don't have a vacuum leak. Does anyone have a more discriptive or correct explanation of the function of this check valve? Are they anymore ideas out there? As I previously mentioned, I am presently able to drive with the boost accumulator/slave bypassed and the brakes, with increased unboosted pedal pressure, work fine.