I've always struggled trying to get my Bonaldi vacuum booster equipped brakes to bleed properly - double the trouble with the dual booster systems circa 1969.
With the help of Papajam I think I finally understand how they operate:
Vacuum boosters all work about the same. The large diaphragm "amplifies" the pressure applied to the brake fluid. This occurs when the front chamber is at atmosphere and the rear chamber is at vacuum creating an pressure imbalance.
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The mystery is how does the system create the imbalance.
When you press on the pedal fluid is pushed into the booster chamber. The majority of the fluid travels down the barrel to the outlet but there is another port that goes to the “compensator valve” (or whatever it’s called). The fluid that goes to the port pushes on that little valve which in turn pushes on the small diaphragm post which presses open the spring loaded valve inside the vented “top hat”.
(sorry, this is upside down from in vehicle orientation)
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When that opens up it vents the front chamber of main, big diaphragm via the outboard tube and that 90-degree rubber tube. Atmosphere applies pressure to the diaphragm which moves the plunger on to the booster piston, closing off the flow of fluid and forcing the brake fluid on the backside of the piston out the outlet at high pressure.
Release the pedal, the compensator valve closes and the outboard tubing circuit goes to vacuum, balancing the front and rear chambers and the plunger is returned to retracted position with the help of a big spring inside the rear chamber.
If true it explains why it is so hard to bleed the system. It all comes down to that dang little compensator valve.
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I don’t see how you could ever get the air out of the little chamber behind the little chamber. And with air in the chamber there is no way the valve will move out and activate the valve to let air into front chamber. IE, no boost.
The only way I can see to properly bleed the system is to remove the compensator valve altogether and press the pedal so to force fluid out the little valve. Then reinstall everything and pray…
With the help of Papajam I think I finally understand how they operate:
Vacuum boosters all work about the same. The large diaphragm "amplifies" the pressure applied to the brake fluid. This occurs when the front chamber is at atmosphere and the rear chamber is at vacuum creating an pressure imbalance.
The mystery is how does the system create the imbalance.
When you press on the pedal fluid is pushed into the booster chamber. The majority of the fluid travels down the barrel to the outlet but there is another port that goes to the “compensator valve” (or whatever it’s called). The fluid that goes to the port pushes on that little valve which in turn pushes on the small diaphragm post which presses open the spring loaded valve inside the vented “top hat”.
(sorry, this is upside down from in vehicle orientation)
When that opens up it vents the front chamber of main, big diaphragm via the outboard tube and that 90-degree rubber tube. Atmosphere applies pressure to the diaphragm which moves the plunger on to the booster piston, closing off the flow of fluid and forcing the brake fluid on the backside of the piston out the outlet at high pressure.
Release the pedal, the compensator valve closes and the outboard tubing circuit goes to vacuum, balancing the front and rear chambers and the plunger is returned to retracted position with the help of a big spring inside the rear chamber.
If true it explains why it is so hard to bleed the system. It all comes down to that dang little compensator valve.
I don’t see how you could ever get the air out of the little chamber behind the little chamber. And with air in the chamber there is no way the valve will move out and activate the valve to let air into front chamber. IE, no boost.
The only way I can see to properly bleed the system is to remove the compensator valve altogether and press the pedal so to force fluid out the little valve. Then reinstall everything and pray…
