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Welcome to our knowledge base of totally random and probably obscure stuff relating to GM THM400 (or TH400) mounted parking brakes! We've recorded these notes and observations not only for ourselves (because we forget stuff over time), but also in case anyone else can find it useful. If you find something incorrect or wish to contribute something, just let us know here: Contact us.

 Parts breakdown
 Adapting from later style






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Parts breakdown

This page refers mainly to the earlier style 1970's and 1980's medium truck/RV transmission mounted (prop shaft) parking brakes. Later 1990's styles used a somewhat similar design, but with a smaller diameter and wider shoe for approximately the same braking area. The actuator was also different being a rod instead of a cable.

Since these parking brakes are becoming somewhat rare and unknown, it is helpful to have a (although incomplete) parts reference. Below is the brake assembly with the drum removed. To remove the drum, remove the driveshaft and take out the bolt in the center of the drum. Note the adjustor wheel can face either left or right, as the access hole in the drum needs to be rotated into position anyway, so the side alignment does not matter; the wheel will be accessible either way it is installed.
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Part numbers we've been able to locate:
   -Parking brake cable: from brake to transmission bracket, Raybestos # BC93510
   -Shoes: AC Delco #171-772
   -Strut: AC Delco #00364694
   -Drum: AC Delco #177-1035
   -Rebuild kit (shoes, springs, cups, pins, washers): AC Delco #179-2135
   -Backing plate: AC Delco #18060000


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Adapting from later style

The later style, which is intended for use with the 4L80E transmission, uses a pedal actuated brake rather than a lever. The actuation system is different, but this style brake can be used with the parking brake pedal. Since both the lever and pedal both use a cable, the cable end may be adapted onto the older style cable. When doing this, allowance must be made at the junction for adjustment due to brake wear over time, because the pedal does not have the wear adjustment built in like the knob on the end of the lever. The easiest way to accomplish this is to use a threaded rod where either cable can be threaded in or out to allow adjustment. The star wheel allows some adjustment on the brakes themselves, but the cable should always have a method of adjustment since other parts of the overall system may wear or stretch as well.

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