Last season my brakes worked well. This year after a moderate cam swap, (about 10Hg of vacuum), the boost goes away on the second or third pump of the pedal when coming to a complete stop, feels like stepping on a cement block. The first pump feels fine. Could my brake booster be bad? The RPM does increase 100 to 200 when applying the brakes, did this last season as well.

Thanks,

Mike

 

Assuming you actually have 10"Hg at idle, that is not enough for a booster to function properly. When you step on the pedal a valve on the back of the booster where the rod enters opens. This sets up the pressure differential between the front of the booster chamber and the rear chamber. This is the "woosh" you hear when you step on the brake. The "woosh" amplitude will vary depending upon how fast and how far you push on the pedal. When you apply the brakes and hold them, the valve should close since it is sensitive to motion. If you hear vacuum coming from the booster with no motion of the pedal, the valve has gone bad and will cause the idle RPM to rise since this is now a vacuum leak. There is also a check valve on the vacuum line that plugs into the booster. Take the valve off and see if it still closes in one direction, but not the other. You do this simply by first sucking on the supply side, which should flow, then blowing into it, which should block air flow. If it allows air flow in both directions, that valve is bad and your booster can't maintain vacuum. You should also have a couple of assisted pedal applications once you turn off the engine. If you don't, then you have valve or booster issues.

Getting back to the idle vacuum; most boosters will need a couple more inches of vacuum to function minimally in their design parameter. Not to say that 10"Hg won't work at all, but you'll suffer a lack of assist in low-speed situations such as you would encounter in traffic. But if everything is sealing properly, it won't affect the idle. Your brakes will just suck.

 

Thanks Gerry good info for sure. So if I understand correctly there should not be a rise in rpm when applying the brake.

 

There will be some affect on idle rpm only because the vacuum chambers are replenishing, but this shouldn't be noticeable and when it occurs, it should be very brief...like a split second and less then 100 rpm. Even on a fully-functional system, you can force a higher idle by pumping the pedal like a madman to deplete the vacuum. You're just creating a vacuum leak by doing this and once you stop pumping, it takes but a few seconds for the chambers to be pumped down to normal. But if the idle speed changes and stays higher while you're on the pedal, you have a problem in the booster valves.

 

Thanks, looks like I will be changing mine. Are the aftermarket rebuilt units from the local parts chains ok to use?

 

I'd make sure of a thorough diagnostic path before I started throwing money at the problem. I use a hand vacuum pump and pump the booster down and see if it holds vacuum and to help sort out the problem. Rebuilt units can be hit or miss. I've had rebuilts that were non-functional out of the box. Quality control leaves much to be desired on the assembly line. I suppose you could take it back if it tested bad or you could farm your original out to Booster Dewey. But, again, make double damn sure you know you have a bad booster.

 

So a simple check with a vac pump and it should hold? If not its bad? Sounds easy enough.

 

This test should tell you if the booster is still good if it holds the vacuum.

At 10 hg you are pretty low. 15-16 is about the minimum I have been told you need for good pedal feel. You will most likely have to add an electric vac pump to get the pedal pressure right or convert to a manual master with a smaller bore.