Brake pedal--hissing......

[knac1234]

Hello all,

So after being stranded with a dead battery today, moved right along in the Series III and noticed when I pushed the brake pedal a loud hissing sound.

It seems to brake fine, but whenever I press the pedal, hissing sounds.

At home, I ran the engine with a piece of wood lodged against the brake pedal to obtain pressure and the hissing sound. I see no evidence of a leak at the brake booster.....but how should I tell?

-The 90 degree elbow is tight with no play
-Maybe the hose needs replacing (will get some 3/8 hose tomorrow)
-I checked the allen screw that plugs the hole in the brake booster---nice and tight

Is there anything else to check? The brake booster and master cylinder were all replaced new in 2008, about 7000 miles ago (prev owner/looked at receipts). I would hope they last longer than that, but anything else???

Cheers,
Julian

[bkreutz]

If the hissing only happens when you press the brake pedal I would suspect the diaphragm has a leak, if it was one of the hose connections or plug the hissing would be all the time. Usually what happens is when the pedal moves the diaphragm a crack is uncovered and starts to leak and when the pedal is released the crack(or tear) is sealed off by the rubber folding over on itself sealing off the leak. Theoretically the diaphragm can be replaced but I remember a discussion a short while ago that said the part isn't available separately and the only option is replacing the booster.

[knac1234]

Gale-

As always, thanks for the quick reply!

Is it safe to drive the truck (I assume it just requires more foot pressure on my part?).

Will wait to hear more on this topic. Not sure where this diaphragm is....perhaps there is a decent used one floating about?

Can't believe the lifespan is so short if it turns out this is the part needing replacement!

Cheers,
Julian

[73series88]

mines doing the exact same thing
definately have to push on the brakes harder.
i just dont want to dish out that money for a new one.
aaron

[SafeAirOne]

Here's an explanation of what goes on inside a vacuum brake booster (servo). What you describe could be a faulty vacuum valve or a hole in the diaphragm.

[Terrys]

If you step on the brake when you are at idle, do the revs change?

[knac1234]

Mark-Thanks for the link.....helps to visualize what goes on inside that part.

Terry-Yes, when I press the brake pedal at idle, my rpm increases by a few hundred or so.

Thanks,
Julian

[knac1234]

So any more input on anything else to check?

If not, how involved is it to r&r the booster? The master cylinder is bolted to it, so I suppose that involves draining and refilling it and then bleeding the brakes? Is there a brand better than another (this one, with few miles and life on it, was from AB).

Thanks,
Julian

[NickDawson]

Check the tiny screw at the bottom of the outside of the booster. If it has fallen out (or rusted away ) then you might get a hiss (although probably lack of pressure as well).

I drove mine sans booster for 6 months. It was relatively fine, like you said, it just requires more foot pressure.

I got the procomp replacement and have been very happy - $ well spent.

[knac1234]

Nick,

Thanks. This screw----I might have checked it already. Looking at the brake booster on the master cylinder side, at about 8 oclock, there is a small allen screw. Is this it or another elsewhere hidden?

Procomp sounds good! Does it require brake bleeding to swap out (never done a booster before, although it looks easily accessible)?

Cheers,
Julian

[Les Parker]

PLF314 Servo

Here's the link to the Proline servo (booster) :-

http://www.roversnorth.com/store/p-4891-brake-servo-assmembly-series.aspx

[SafeAirOne]

Here's the link to the Proline servo (booster) :-


http://www.roversnorth.com/store/p-4...ly-series.aspx

I furnished that link to him in a PM, but he says RN qoted him a significantly higher price than that shown in the link.

[bkreutz]

Nick,

Thanks. This screw----I might have checked it already. Looking at the brake booster on the master cylinder side, at about 8 oclock, there is a small allen screw. Is this it or another elsewhere hidden?

Procomp sounds good! Does it require brake bleeding to swap out (never done a booster before, although it looks easily accessible)?

Cheers,
Julian

If you don't disconnect the brake lines from the master cylinder, no bleeding is required. (there's usually enough slack in the lines to unbolt the m/c and pull it forward enough to get the booster out)

[SafeAirOne]

Check the tiny screw at the bottom of the outside of the booster. If it has fallen out (or rusted away ) then you might get a hiss (although probably lack of pressure as well).

A leak on the frontside of the servo, where that screw (it's really a plug in a threaded port) is located should produce a constant hissing as that side of the diaphragm ALWAYS has vacuum going to it.

Brake boost is derived by blocking vacuum off on the backside of the diaphragm and venting the backside chamber to the atmosphere, thereby allowing the vacuum (still present on the frontside of the diaphragm) to "suck" the diaphragm forward, helping you push the piston on the master cylinder.

I understand that the port with the plug that you refer to was where a vacuum switch was located on some models. Loss of vacuum would trip the switch and illuminate a "brake failure" lamp on the instrument panel.

[knac1234]

Gale--Good news, should be easier to replace.
Mark--Were you referring to someone else, as I received no PM?

Julian