Frozen Brake Cylinders

A little advice is needed regarding bleeding a dry brake system on my 120. After rebuilding all the wheel cylinders, because they were all frozen, I went to try to bleed the system. The problem is that since it is a new master cylinder, the seals are too tight to let the piston return under its own force to the position where the tilt valve will open. Because I can't fill the master cylinder, I can't get any fluid into the system. I've tried to use a vacuum to draw the fluid from the master out through the bleed nipples. But I was unable to get anything. Any ideas as what to do? - Aaron Pincus

Aaron: Drain the reservoir, disconnect the brake lines from the master cylinder and blow the master cyl pistons back to the retracted position using air pressure. Leave the reservoir cap on during the blow-back process since there may be a small geyser when the tilt valves open. When next you try to bleed the system, consider applying modest pressure (air) to the reservoir (10psi), when you open a bleed screw. You may have to make an adaptor to fit the threaded reservoir opening to apply the air, or as a minimum, if you have a rubber tip on your air hose nozzle, you may have a helper apply some air directly through the reservoir cap vent hole. I hope you're not using silicone fluid. If so, the natural rubber seals may have swollen and the pistons may never move freely. That being the case, you may want to remove the master cylinder now, clean all the rubber seals in alcohol and switch to Dot 4. If the silicone fluid has gotten to the wheel cylinders the sticking problem could also occur there. Good luck, several of us have been there before. - Dick Cavicke, 120 OTS & FHC

Also try tapping (lightly) on the master with a light hammer. I've had good results using this method. The tapping adds a little mechanical shock to the system and helps allow the internal springs move the piston past a high friction point. Cleo Bay, XK120, Xk140

Is this true (silicone) with new wheel cylinders?? I have been using for several years without a problem. Thanks - M. Larsen

To all: Somebody might have already suggested this test but I can't remember ever reading it. Since the two camps (yes silicone) and ( no silicone) will never agree, I propose the following: since its impossible to know the type of rubber used - silicone compatible, or not silicone compatible - pretest the seals in a jar of silicone and see if they swell up. As a control put some extra seals of the same type in a jar of DOT 4 and compare. The 120 and 140 seals are inexpensive, generic, and available at any parts supplier. Pretest a set of seals and install them if they don't swell. I am a strong supporter of silicone, and I have never had any problems, maybe I was just lucky that I always had silicone compatible seals. Regards. - Wray Schelin

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