Interesting Hydrovac observation...

[pfarber]

This happened to me:

from http://www.binderbulletin.org/forums/sh ... hp?t=77210

"The main failure point on boosters like yours is the piston rod seal which leaks brake fluid into the diaphragm like yours did. I bet you were losing brake fluid with NO visible leaks. ie the master cyl. would be low or empty after sitting awhile, what was in yours may look like oil but I bet it was brake fluid from the seal leaking for a long time. Sometimes when they get to much fluid in that area, when you hit the brakes the vacuum will pull the fluid into the intake and make the truck smoke."

The big tell is loss of fluid from the master cylinder after sitting. The seal is not a big one physically, but probably the most common to go. I'll dig up the PN later and post a pick of the seal.

[MotorpoolMac]

A trick I learned, is to pump the brakes three of four times when you turn the engine off. This lets off the vacuum in the hydrovac and keeps the vacuum from sucking brake fluid while the truck is setting. If your seal is just a little wore you wont notice much loss of fluid from limited operation ...but setting with that vacuum in the hydrovac will suck fluid out of your slave cylinder every time you park it.

Daniel

[pfarber]

The vacuum would normally bleed off in a few hours anyways (via the intake check valve most likely).

Correct me if I am wrong, but pumping the brakes would admit atmospheric air to the 'back side' (RED) of the pistons and the vacuum side (BLUE) would not really 'release'.. just be compressed. So you would have a fully engaged piston and piston rod still having a vacuum on it the seal..

vacpos1-bw.jpg (11.94 KiB) Viewed 8188 times

Now you would have vacuum on the seal PLUS the spring wanting to return the piston to mid point. As long as your intake check valve holds, you will have a vacuum on one side of the piston.

[MotorpoolMac]

There is a tube that connects both sides of the vacuum cylinder to the line to the manifold. Therefore vacuum is delivered directly to the Blue side (in your diagram) and through the control valve on the Red side.

Here is a link to an original Army training film explaining the operation of the hydrovac system:

http://www.youtube.com/watch?v=keChCevuKk8

When the engine is not running we have only a limited amount of vacuum. Pressing the pedal allows atmospheric pressure to reach the Red side as you described, but when the pedal is released the control valve shuts off atmospheric pressure and opens the Red side to the vacuum line again. This is in practice "connects" the Red side with the Blue side.

If the engine were running the vacuum would clear the Red side of pressure while keeping the Blue side clear, returning the system to equilibrium. However, if the engine is NOT running when the pedal is released, the control valve again “connects” the two sides and the atmospheric pressure on the Red side then "contaminates" the vacuum on the Blue side.

Pushing the pedal again allows more atmospheric pressure in to be let into the Blue side when the pedal is released until all the vacuum is gone.

[deadline]

I am going to link that video to the tech pub section.. thanks for digging it up.