Hissing brake pedal (1 Viewer)

[Slow Poke]

Just put a lift on my 77 fj40. In the process I had to change out brake lines and while I was at it I thought I'd paint that ugly rusty master cylinder. All back together now and back from a test drive. My brake pedal is soft and continues a slow descent when pressed. The whole time it's descending I here a hissing noise from the brake pedal area. Does this mean my booster is bad? It seems odd that it was fine before I started messing with everything. Do I just need to bleed the breaks again?

 

[MtnClimber]

Vacuum leak within the booster. Try seeing if you can disassemble and see what might not have assembled correctly.

 

[FJforty]

How far down does the pedal go? The MC will make noise if your pedal continues to go down. The MC pulls a vacuum on one side and air rushes in on the other side of the rubber membrane.

 

[65swb45]

Just put a lift on my 77 fj40. In the process I had to change out brake lines and while I was at it I thought I'd paint that ugly rusty master cylinder. All back together now and back from a test drive. My brake pedal is soft and continues a slow descent when pressed. The whole time it's descending I here a hissing noise from the brake pedal area. Does this mean my booster is bad? It seems odd that it was fine before I started messing with everything. Do I just need to bleed the breaks again?

Don't overlook this possibility: perhaps you put a crack in a 30 year old brittle hose or the check valve while you were r&ring the m/c?

Best

Mark A.

 

[Slow Poke]

I didn't get a brake line tightened down enough. I was weeping brake fluid down my right front wheel. It still whooshes when I step on the brake but its holding firm now. Time to re-bleed the system.

 

[Fast Eddy]

I didn't get a brake line tightened down enough. I was weeping brake fluid down my right front wheel. It still whooshes when I step on the brake but its holding firm now. Time to re-bleed the system.

My money is on a bad booster causing the whoosh. Still have the original one? Both the ones I've had have gone bad during brake work.

 

[Slow Poke]

yep, original. No one else's booster sounds like this? I didn't notice it when the brakes were adjusted correctly.

 

[numby]

After shutting off the engine wait a minute or so and depress the brake pedal. You should have enough residual vacuum in the booster for one last assisted stomp. It should make a hissing sound. The pedal feel should get rock hard by the third stomp (if not the second). If there's a vacuum leak in either the diaphragm or hoses then the pedal will be hard on the first application of pressure.

 

[Slow Poke]

OK, did the Numby test. I seem to have some assist after a minute or so and it takes about 4 depresses of the brake pedal for the hissing to stop. I still hear a noticeable hiss every time I use the brakes when I'm driving. It stops great and I think I'm getting enough assist but I'm worried I may have a vacuum leak. This is my first fj experience so I'm not sure if this flat tire noise is normal.

 

[65swb45]

OK, did the Numby test. I seem to have some assist after a minute or so and it takes about 4 depresses of the brake pedal for the hissing to stop. I still hear a noticeable hiss every time I use the brakes when I'm driving. It stops great and I think I'm getting enough assist but I'm worried I may have a vacuum leak. This is my first fj experience so I'm not sure if this flat tire noise is normal.

A temporary whoosh is ok, a constant one is not. And a whoosh accompanied by a stumble in the engine idle will certainly end up hurting the engine if left uncorrected.

Soooooo.....does it stumble?

Best

Mark A.