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GM 3 line Brake System Information
#1410378 Tue May 18 2021 08:48 PM
Joined: Nov 2019
Posts: 83
HFfarms Offline OP
Closing the loop on this for all future readers/references.
Original discussion with info is here ->
Summary and update posted here in new thread with more easily searchable title and consolidated information for easier reference.

Situation summary:
My truck (1964 C60) has a factory 3 line brake system comprised of a larger 12.75" diaphragm hydrovac (PN: 2501850), larger 1.25" bore master cylinder (PN:5462223), and a 3rd line. Both primary lines are 1/4" with the 3rd line as 5/16". This was a factory available "upgrade" this is not common as most trucks came with the standard 2 line system with 11" hydrovac and 1 1/8" master. There is a single line running from the tip of the MC to the slave cylinder (front) of the HV, a second line from the back of the HV in thru the frame to the wheels, and the third line from the very piston tip of the HV, back to the MC where it enters the passenger side of the reservoir. This 3 line system was an upgrade to the smaller 2 line system with increased braking performance. Due to the rarity, parts are near impossible to find. I struck out repeatedly for rebuilt kits and rebuild capability. Even struck out with custom shops to professionally rebuild both HV and MC due to lack of parts/info available for this system.

I just got off the phone today with Mr. Tim Henderson at Precision Rebuilder in St. Claire MO who has worked on brake systems for medium duty trucks for the last 40+ years. I merely mentioned "1964 3 line system" and he rattled off the part numbers for the HV and MC from memory. The discussion that commenced from there I will capture a brief summary of.

The system was introduced around ~'61 and only ran thru '64. The system as he described it, is a "problem child" that is plagued with issues and malfunctions. It just wasn't engineered very well and is weak in comparison to equivalent systems, thus its short factory life from '61-'64. The HV has a much longer and larger piston than comparable units thus the requirements for a larger MC and more fluid to operate, and the third line is what recovers the system during operation. Further, the entire point of the 3 line system is to enable operation of a large HV while utilizing small bore MC's (1.25" bore MC is still very small compared to the piston size of the 12.75" HV). When this system was dropped at the end of '64 model year, all '65+GM trucks switched to 1.5" bore masters for this reason. Lastly, the 3 line system is a bear that is nearly impossible to fully/properly bleed. If the fluid returning to the reservoir shoots into the reservoir under any amount of pressure, then there is air in the system. It is supposed to slowly trickle in. Bleeding to the point of no air in the system is reportedly, next to impossible depending on situation, due to the design of this system.

Precision can rebuild both the HV and MC, but thru discussion with Mr. Henderson, I opted against it. I would rather have a reliable system on hand for safety reasons, and eliminating the 3rd line seems line a very acceptable simplification. Mr. Henderson will take my current MC and HV as core as well as blueprint for custom building a new 2 line system on a 1.5" bore master that should perform similarly to this "factory upgraded" system. Recognizing the wealth of knowledge Mr. Henderson possesses from the years of work on these systems, I picked his brain some more. Turns out, I have been bleeding the brakes on my truck wrong, and what he described I have yet to see written out anywhere, including factory service manuals. In short:

The engineers who designed/built the Wagner type rear axle brake system cocked the upper cylinder such that unless the shoes are fully adjusted inward with the manual adjuster, the bleeder screw is not the highest point, the expanded shoe cup is. Thus, any air bubble will sit in the expanded cup and not be able to be bled out. The correct way to bleed the brake system of these trucks is to adjust the shoes all the way inward, bleed the rear axle, and then manually adjust the shoes outward. The only other option is to lift the rear of the truck until the bleeder is the higher point, in the 12:00 position, which is approximately the height of a 55gal drum.

I wanted to post all of this information as I know there are other 3 line system trucks out there and most here had never even heard of this system when I brought it forward a little over a year ago. Should you be one of the few with this system, call Tim at Precision rebuilders and speak directly with him, he is the all-knowing that can answer your questions.
800-325-2690 Tim Henderson Ext. 109

I hope this helps.

Re: GM 3 line Brake System Information
HFfarms #1410405 Wed May 19 2021 12:02 AM
Joined: Nov 2005
Posts: 5,936
3B Offline
Hy HFfarms, Thank you so very much for the update and the description of a braking system I had never seen before. As you said, I had never seen the bleeding instructions you gave us written in any maintenance manual. I have heard some of my colleagues talk about having to raise the rear of the truck high on the hoist to get the system to bleed properly. Thanks very much for the information!

Re: GM 3 line Brake System Information
HFfarms #1410579 Thu May 20 2021 04:29 AM
Joined: Apr 2020
Posts: 156
HFfarms another way to bleed them is to leave dust boot loose and with thin feeler gauge slide past cup to let trapped air out

Karl j Townsend

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