Set up - Power brakes - front disc (factory). Booster replaced with rebuilt about a year ago. Due to low vacuum (less than 12hg from engine), there is an electric vacuum pump which is currently producing about 19 hg constant (no cutoff switch). Master cylinder full and fluid clean. No leaks seen near master cylinder.
A few weeks ago, I lost power assist coming off the highway. After getting home, I found that the hose from the vacuum pump to the brake booster had dropped down and contacted the power steering pulley which rubbed a hole in the hose. I replaced the hose, however it seemed like the power assist was not working. I tested the vacuum at the end of the hose going from the vacuum pump to the booster and it read 19hg, which should be sufficient for brake assist and indicated the replaced hose was operating at expected, but it still felt like a hard pedal and would not stop like it did prior to the hose issue.
I finally got back to it today and tried a couple of garage tests. I let the pump run to build vacuum and tried the pedal. It seemed to have some assist and the pitch of the pump changed when I would step on the brake as if it were rebuilding vacuum. I also did the pump and hold test, which was inconclusive (sometimes seemed the pedal moved, other times it did not). I also tested the pressure valve by removing it and there was a hiss of air as it came out.
I took the Cougar out for a test drive in the neighborhood (very slow speed and low engine RPM) and at first the assist seemed to work, then it would not. Then it would work again, then it would not. The difference seems to be the time between using the brakes. A longer duration (30 seconds between braking) seems to use power assist, where a shorter (10 seconds) time between braking seems to not use the power assist.
I think the power booster is the culprit, but I don’t want to go through the time/expense of replacing/rebuilding it if the issue is not the booster.
Any advice is greatly appreciated.
Im thinking dwbeis from the chafing of the hose in the original failure are causing the intermittent failures. Clean it out stem to stern.
Mike M.
I appreciate the input!
That is something I thought about, but not followed up on. The damage was near the pump end of the hose not close to the booster. My thinking is that the debris should have been pulled into the vacuum pump as that is the direction of the pressure. In that case, wouldn’t the vacuum from the pump be lower than 19 and also fluctuate due to debris/damage from debris? The vacuum pump has an ‘exhaust’ outlet and the flow seems steady and reasonable in pressure which is (to me) another indication that it is functioning as expected.
I think I need to take a close look at the hoses/fittings to see if there is a way that debris could have traveled towards the booster instead of away from it. It probably would not hurt to replace all the related hoses and fittings (less expensive than a new booster/vacuum pump).
Are you still using the vacuum reservoir? Is the check valve still in the line? Not sure if the check valve will do any good with a vacuum pump but it could be obstructed with hose debris.
I need to add some detail to the entire set up. I’ll work up a drawing tonight to help make sense of this.
Everything except the brake booster are still being run from engine vacuum (headlight switch/doors/reservoir, transmission, timing advance). That all seems to work fine. The only difference from stock is that the vacuum tree on the firewall is not used. All hoses run directly from the vacuum ports on the intake manifold.
However, the two systems are not isolated. There is a junction at the TEE between the pump/booster/manifold. The inline check valve off the manifold to the TEE seems to work as expected, only allowing flow of air towards the manifold creating a weaker vacuum towards the brake booster (I presume this is a safety feature in case the vacuum pump were to fail…a little boost is better than none).
Vacuum pump ← TEE ← Brake booster (with check valve that seems to work as expected)
Ports on manifold ← inline check valve (seems to work as expected) ← TEE (same from above)
I hope this makes sense. I’ll dust off my tablet drawing skills and come up with a graphical representation tonight.
This does make me think that once the hose from the vacuum pump started leaking, debris could be pulled towards the engine vacuum ports. I checked the hoses and fittings with air and nothing came out (other than air), so there does not appear to be any residual debris in the lines. The check valves are operating one way only as expected. If debris has entered the booster, then the booster would need to be replaced/rebuilt as it could have damaged the diaphragm. If debris has entered the manifold ports, that switch would need to be cleaned out/replaced. However, since that is isolated from the brake booster when the vacuum pump is running, it should not have any impact on braking.
