7.5 - Holding Solenoid Repair- Rear Brakes Locking On ( Tom Brady,  December 11, 2005 )

I want to share the following experience with all MK IX owners with automatic transmissions who have been plagued with brakes locking on or dragging. I have had the problem intermittently with my 1959 MK IX, which I have driven about 5000 miles over the past two years.

Three years ago I rebuilt the brake system as the car was being restored, including the following:

1. New brake booster (the new replacement type offered by SNG Barratt, not a rebuilt original)
2. All brake cylinders resleeved in stainless steel
3. All new seals in the brake cylinders
4. All new stainless steel brake lines
5. New brake fluid reservoir
6. Silicone Dot 5 brake fluid

The master cylinder appeared to work OK, as well as the holding solenoid. However, I noticed the brakes took two or three seconds to release after I applied them. I learned to live with it, hoping they would work themselves free with time. That did not happen. Last summer, the brakes (front and rear) dragged to the point of smoking, with the pedal becoming fully locked on. I parked the car, waited a few hours, and the brakes freed up. I could now move the brake pedal. I drove the car about 12 miles home and had the same situation occur again about a mile from home. Again, after an hour, I was able to make it the rest of the way home to my garage.

I was convinced the problem had to be the master cylinder, which I had not rebuilt. I removed the master cylinder and cleaned it thoroughly, and replaced the three rubber seals. I noticed a separate spring loaded check valve that engages and seals off the brake fluid reservoir as the piston in the master cylinder is depressed. That certainly made sense: otherwise fluid would be pumped up to the brake fluid reservoir. I was convinced that the crud in the master cylinder was somehow preventing the piston from returning, resulting in dragging of the brakes. I refitted the master cylinder and test drove the car. The rear brakes heated up once again. But the front brakes were cool. Voila, the holding solenoid had to be the problem.

I disassembled the holding solenoid (not an easy task) and found plenty of guck that was preventing the piston from moving. The theory of operation is as follows:

1. At rest, with no car motion, the solenoid coil is energized by a mechanical switch mounted between the carburetors. The solenoid plunger, which has a rubber seal on the end of it, closes off the main fluid passage to the rear brakes.
2. If the brakes are applied when the solenoid is energized (as occurs while coming to a stop), the brake fluid pressure is held by the check valve and the plunger. It works as follows: brake fluid is allowed to pass through a second set of four smaller holes that allow flow only to the rear brakes. A spring loaded disk covers all four holes and acts as a check valve, allowing flow of brake fluid to the rear brakes.
3. When you step on the accelerator, the mechanical switch opens, the coil is de-energized, and the rear brake fluid pressure is released.
4. If the car is in motion, a pressure switch, mounted on the transmission in series with the mechanical switch, opens the coil circuit and prevents the coil from energizing.

There are more details to the operation, but the basics are described above.  I intend to write a paper on the holding solenoid rebuild details and will share it with anyone who wants a copy.

I refitted the solenoid and the brakes now release immediately. I know that some (maybe most with automatics) Mk IX owners have had the same problem and I have seen it blamed on silicone fluid, a bad brake booster, etc. I just wanted to share my experience, because it took a long time to sort the problem out. Some owners may simply want to take the holding solenoid out of the circuit and replace it with a short length of straight tubing. I actually like its function, holding the car at stop lights and on hills. To each their own. I think it is a pretty ingenious device.