Old truck with air-assisted hydraulic brakes coupled to a modern air-brake trailer

[Robo McDuff]

I really searched, some answers came close, but never really found a satisfactory THEORY answer I could understand.


How does an old air-assisted hydraulic braked truck regulate the brake power of a modern trailer with air brakes?




My 1973 M51A3 has air-assisted hydraulic brakes which I plan to convert from a single into a dual system. When trying to figure out what new MC to use and double air pack or hydro-boost support, I started wondering about the trailer.

I now how air brakes work in all details, and the same for xxx-assisted hydraulic systems. But how do these older truck brake pedal-mc-airpack systems regulates the air pressure to the trailer brakes so that even when hooking up a modern 20+-ton civi trailer it balances and the trailer does not over or under-react?

 

[gimpyrobb]

So your asking how the air pack on our trucks work? Are you familiar with how a vacuum assist works?

Diesels do not make vacuum, so an air compressor was installed. Instead of vacuum pushing the brake piston, you have air pressure pushing on the piston. They are exactly the same, except on pushes and one pulls.

 

[gimpyrobb]

Best I could find quickly:

 

[gimpyrobb]

This is an informative site. Scroll down to "hydroboost".

http://www.gonzostoolbox.com/KnowledgeFolder/BrakeBooster.html

 

[Robo McDuff]

So your asking how the air pack on our trucks work? ...

No. During my CDL courses in the 1980s all brake systems were extensive explained: simple hydraulic, vacuum-assisted hydraulic, air-assisted hydraulic (on our MVs), air-actuated hydraulics and full-air. Still have that book and when checking for my refresher course just now and for the work on my M51 I went through it again. The 1982 book explained that the trailers with air brakes reacted directly on the air brake pedal of the truck. Newer trailers (that is late 1970 early 1980s versions) also had load-dependent reaction valves that helped regulate the brake pressure applied to the trailer brakes.

However, the system did NOT explain if that connection worked as well with an air-assisted brake system.

I rechecked another thread for pics, and found the link back to the Air Brake Manual (link to the thread). The article can be downloaded at http://www.mpi.mb.ca/PDFs/AirBrakeManual/ABM_Section2.pdf.

Page 21-23 give a good summary. With an air-actuated hydraulic brake system, the brake pedal activates an air valve and the connection to the trailer is behind the pedal (foot valve, page 22). Actually, in a dual brake system, both full and air-actuated, the valves steering the control line to the trailer get their air pressure from both lines (circuit 1 and circuit 2) so if one of the two goes out, that part of the vavle closes and the commands come only thorugh circuit two and vice versa or from both if all is ok.

With an air-boosted hydraulic system, the brake pedal acts on the master cylinder, and then to the air pack which boosts the hydraulic output (page 23). Take also a look at the TM 9-2320-211-20, page 2-161, fig 2-188. You see that the emergency circuit (left lower and right upper Glad Hands) get its air directly from the tanks. The other - control circuit connects to the air pack, but it is not clear to me how the how air pack helps regulate the pressure in the control lines to the trailer.

The air pack rebuild manual gives a good view as well, but that point is not mentioned either.

Now a modern trailer presumably has a load-reaction valve. However, as far as I can see, the M51A2 does not have any of those things, neither a trailer regulation valve nor a load-reaction valve. So coupling this ancient system to a modern trailer, how does the system makes sure that the applied AIR brake pressure in the trailer matches the air-assisted HYDRAULIC brake pressure in the truck regardless of the load?

 

[5tonman1971]

I think it should work fine I just towed my m923a2 with my 71 m813a1 with the tow bar connected and had the glad hands connected emergency and service and the 813 was controlling the 923s brakes just fine and it is a modern air brake system.