Welcome to our knowledge base of totally random and probably obscure stuff relating to GM THM400 (or TH400) transmission-mounted parking brakes (also called prop/propeller shaft 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.
 TH400 Parking brake TH400 parking brake parts breakdown
 TH400 Parking brake TH400 parking brake adapting from later style
 TH400 Parking brake TH400 parking brake tailshaft housing
 TH400 Parking brake TH400 parking brake on factory overdrive unit
 TH400 Parking brake Resources
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TH400 Parking brake 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 (but the directon to tighten/loosen will be reversed). In the first photo below, the adjusting wheel is shown in the 'proper' position whereby turning the wheel 'down' (from outside through the drum access hole) will result in tightening the brake against the drum.
TH400 parking brake picture 1 TH400 parking brake picture 2

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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TH400 Parking brake 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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TH400 Parking brake tailshaft housing


These units have a unique tailshaft housing, which contains two separate ball bearing units that support the tailshaft/drum assembly. Another unique feature is the four bolt holes, and usually an aligning pin (unless it has been broken off or punched out) to mount the brake backing plate. The tailshaft adapter housing also has a lip around the outer edge to position to the backing plate. When the drum is fitted over the splined output shaft and seated against the backing plate, the inner races of the bearings ride on the outside of the drum/shaft assembly and take the full weight of the unit. The drum is then center-bolted into the end of the output shaft. One or more rubber O-rings seal the joint between drum and shaft. As a note on this: the output shaft and bolt are interchangable with the bolt-on driveshaft yoke used on other TH400 units, but the tailshaft housing is not.
TH400 parking brake tailshaft housing TH400 parking brake tailshaft housing

Part numbers we've been able to locate:
   -Tailshaft housing output shaft/drum seal: National #2457 or SKF#21111. Red seal is for standard tailshafts only, the proper one for parking brake type housing is GREEN, and a larger diameter to fit around the drum shaft.
   -Tailshaft housing output shaft bearings: National #109
   -Tailshaft housing: cast aluminum, one casting # (there are several) is "8627143 LESTER-1"

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TH400 parking brake on factory overdrive unit


In the 70's and 80's, GM worked with Gear Vendors to provide an overdrive unit for the RV chassis that would be directly compatible with both the TH400 and its parking brake system. Many of these systems are still alive and well due to their durability.

The unit is factory, and bolts directly to the rear of the TH400 unit. Notice in the pictures below how the parking brake cable bracket position remains the same, while a tube extension is provided for the cable.
TH400 parking brake tailshaft housing with overdrive TH400 parking brake tailshaft housing with overdrive

These units were later superceded by the 4L80E transmission which contains its own integrated overdrive.

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Resources


We found these resources valuable for reference material of these transmissions, although they do not reference the brake system specifically:

     TH400 B&M Transkit Manual
     TH400 Parts List
     TH400 Service Manual
     TH400 Super Beefing

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