Solved Hard brake pedal with a hissing noise.

[rbbaker]

I think I know what the answer is but I am looking for input from some confirm my suspicion or to correct me.

Here is the back story. Brakes started acting funny on my '92, at first had to pump them a bit and after a few days to a week or so I had to start pushing on the pedal real hard to stop. Found some issues and have replace a wheel cyl, drivers side rear brake line and the master cyl. Master cyl was bench bled, all brakes bled well all 6 or 7 times that I bled them. No leaks. No air in the system. At no time did the system run dry so I can not see the rabs being an issue.

What I am encountering now is a very stiff, almost like manual brakes, pedal and I am hearing a hissing noise coming from what seems like is under the dash. I am leaning towards it being a power booster issue. Do people concur, and if so are there any parts that can be replaced or do I have to replace the whole unit?

2 things I forgot to mention, I do not recall hearing the hissing before I replaced the master and the old master was not leaking back into the booster.

 

[2stroke]

I would agree, the brake booster is the culprit. Take a good look at the rubber hose first and make sure it's not cracked and has a good fit on both barbs. Also make sure the grommet into the booster is ok. It is most likely the diaphragm in the booster itself. Assuming the master cylinder is still ok, you can replace just the brake booster.

 

[rbbaker]

I would agree, the brake booster is the culprit. Take a good look at the rubber hose first and make sure it's not cracked and has a good fit on both barbs. Also make sure the grommet into the booster is ok. It is most likely the diaphragm in the booster itself. Assuming the master cylinder is still ok, you can replace just the brake booster.

Thanks for your response, that is what I was figuring. The vacuum line and connections are fine and the master cylinder is about 3 days old. The hissing is most pronounced under the dash when I push on the brakes. Cannot hear any hissing under the hood.

 

[Centaurious]

Borrow a vacuum tester and adapt it up to the size of the brake booster hose and test the brake booster. If it holds vacuum, great, if not replace it. Also, the purge valve for the cruise control is mounted on the brake arm and might be your vacuum noise but it wouldn't account for the hard pedal.

 

[BrooklynBay]

The easy way to test it is to clamp the vacuum hose going into the power brake booster, then listen to see if there's still a hissing noise. This will rule out the possibility of a hidden vacuum leak inside the power brake booster.

 

[rbbaker]

Thanks all, broke down and spent 25 bucks at a wrecker and swapped out the booster. Brakes work like a charm now.

 

[BrooklynBay]

Thanks for the update. Your thread is now marked solved.

 

[DonQueso]

I'm having literally the exact same issue on my '98 4.0 OHV. I've replaced all the pads and rotors (which needed replacing anyway) and bled all the calipers and the master cylinder (not bench bled, just through the fitting). Thanks for your contribution. I'm ordering a reman booster tomorrow and I'll update on how it turns out. But first, any input on whether I should go AC Delco or A1 Cardone? Those are my two options and they are both remanufactured. The price difference is negligible.

 

[DonQueso]

I went with A1 Cardone. Should be here in a couple of days. I'll follow up with the results.

 

[DonQueso]

Well this was a major win for me. I've successfully replaced the power brake booster after having already replaced all brake pads, rotors, parking brake shoes, and flushing the brake lines with new brake fluid. Funny thing is my foot hasn't gotten used to not stomping on the pedal yet so every intersection is an adventure. Thanks for your help on this, I hope it goes on to save someone else a lot of time and trouble!