What is the difference?

[carmen]

Can someone please explain to me the difference between an air pak or a vacuum booster... brake system. Now I want you to know that I have completed a 1945 MB, and a 1954 M37. So I do understand those brake systems. But looking and reading about the M135, which is what I am working on now, I just don't understand the differences between them. It seems that the more I read the more I don't understand.

Thanks


Carmen

 

[JDToumanian]

It's just a different way to do the same thing, air pressure boost vs. vacuum boost. Smaller vehicles like automobiles, jeeps, M37s, etc. use vacuum boosters because the gasoline engines produce enough excess manifold vacuum that it can be stored in a reservoir and used to assist braking. Larger gas powered vehicles (like your M135) use air boosters because the engine's manifold vacuum is not enough. Diesel engines produce no usable manifold vacuum at all.

So, for larger trucks and any truck with a diesel engine, an engine powered pump must be fitted for brake boosting... Air compressors are definitely the most common, but I have seen bigger trucks that have a vacuum pump instead.

Regards,
Jon

 

[TheBuggyman]

Jon is exactly right. Although modern vacuum pumps are very efficient at drawing vacuum maybe the relationship between 29"hg of vacuum (very good vacuum) and 90 psi of compressed air dictate the diaphragm size. Also consider that most engines at idle may draw 12 to 22 inches of vacuum and even less while under load so that also shows the need for a larger diaphragm.
One benefit of a vacuum is that water will boil and be drawn as a vapor out of the pumping device virtually eliminating a water/corrosion issue.