Heinkel Brake System Part 2- Wheel Cylinder Overhaul

The wheel cylinders contain pistons that push the brake shoes against the wheel hub. If you are restoring your car you’ll need to spend time on these. Taking apart a brake system on a car that has sat for some time can be challenge. The main problems are rust and dried up brake fluid.

The wheel cylinders are held onto the backing plate with two slotted screws. Try the screws gently at first and if they don’t move use a torch and penetrating oil to loosen them up. Use the same procedure with the bleeder valve.

Once you remove them they’ll look like this. 
Heinkel Car Brake Parts and Cabin Scooters
Heinkel Car Wheel Cylinder- View of Inlet and Mounting Holes

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Heinkel Car Wheel Cylinder- Brass Adjuster and Bent Brake Shoe Connector

Some brake parts are unavailable new so care needs to be taken in their disassembly. The brass wheels in the picture are adjusters that contain metal pieces with slots that hold the brake shoes. In the picture above the metal piece is bent and needs to be replaced. The little arms on the metal pieces can be easily broken. If you can’t unscrew them from the brass wheels I suggest them soaking them in denatured alcohol or water for some time to see if you can soften the hardened up brake fluid that is holding them fast.

Once you get them off thoroughly clean the threads. These need to move freely and easily once they are reinstalled or it will be impossible to properly adjust the brakes.

The brass wheels just sit on top of the ends of the cylinders. There’s an O-ring that sits inside that prevents fluid from weeping out. These are usually gone or have disintegrated. In the photo below you can see the vestiges of one on the right hand brass adjuster wheel. After you’ve removed the adjuster wheel, remove the pistons from the cylinders. The easiest way to do this is to blow compressed air through the inlet. You should be able to blow the pistons out of the cylinders.

Heinkel Cars, Trojans and Cabin Scooters
Brake Parts Left to Right- Brake Shoe Holder, Adjusting Wheel, Piston with Seal, Cylinder, Adjusting Wheel

In the cylinder pictured above you can see the piston with the remnants of the seal on the left hand side. The cylinder originally had a piece of spring metal that “clicked” when the brass adjuster wheel was turned. Over time these can break off. If you have this situation you don’t need to worry about replacing them as it will not affect you braking capabilites.

Now you need to evaluate the condition of the cylinder bore. It will probably have dried brake fluid that will need removal. You can soak some of it off, but you’ll probably have to hone the cylinder.

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Brake Hone for Use on Heinkel Wheel Cylinders

I use a hone and drill press to clean up the bore.

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Brake Hone in Drill Press- Secure it Before Starting!
After that I suggest cleaning any remaining rough spots with extra fine wet dry sandpaper wrapped around a deep bore socket  used in a drill. A 7/16″ socket wrapped with 1000 grit worked well for me:
Heinkel Cars and Cabin Scooters
Polishing a Wheel Cylinder with Wet Dry Paper Wrapped Around a Socket
Once this is clean you can reassemble the cylinder. Use all new parts; it’s false economy to try and reuse the old ones. 
You will need the following the parts for each wheel cylinder:
  • 2 rubber o-rings that sit inside the brass wheels
  • 2 pistons
  • 2 piston seals
  • bleeder valve

One thing to be aware of is the orientation of the piston seals The seals are narrower at one end than the other. The wide end should face the inside of the cylinder.

I don’t know what the tolerances are for the bore of the cylinder. It’s possible that by honing the cylinder you will exceed the permissible diameter. I couldn’t find measurements published anywhere, you’ll just have to figure it out.
The other option is to buy replacement cylinders from the German Club. They have them custom made to fit the Heinkel cars. The holes on the backing plate need to be elongated slightly as the mounting holes on the cylinder are about 2mm farther apart. 
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Heinkel Club Replacement Cylinder on Left, Original on Right
That’s it for wheel cylinders. Up next: the master cylinder.
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