Brake booster squeal

[Cuppa]

First up, this is an issue on my Nissan Civilian, not the Patrol, but I’m hoping that there will be some knowledgeable folk who can offer advice.

Some time back I had the brake master cylinder re-sleeved by Power Brakes in SA. After I refitted it I have occasionally noticed a high pitched squeal from the vacuum booster unit when pushing hard on the brake pedal when parked. It doesn’t happen every the time, in fact it can be hard to replicate. Until last weekend it had never occurred when driving. The brakes function well, & pedal travel is within acceptable limits. Last weekend I braked heavily going downhill, to avoid over shooting a turn I needed to take & the squeal occurred.

I assume that the ‘pushrod’ that the pedal linkage pushes into the master cylinder is traveling past a sealing point in the booster allowing the vacuum seal to be breached, & had hoped there might be some adjustment on this ‘pushrod’, but according to the manual unfortunately not. Of course my assumption may be incorrect & the cause something else to do with the brake booster I have not considered/understood.

As I understand it even if I lost all vacuum I would still have some brakes, albeit without ‘assistance’ & this requiring a lot more effort. As it is when I hear the squeal the braking ‘assistance’ has already occurred, so an emergency involving loss off, or insufficient braking is not going to happen. Obviously the problem is not going to get any better & I suspect that replacement of the booster unit will be required down the track. My gut feeling is that it will be safe to use the vehicle as is, but that I will need to pay close attention to any increase in severity or frequency of the squeal.

Does this sound like a reasonable strategy?

Also are brake boosters fairly generic items, or would I need to look for one specific to my bus?

[mudski]

I'm no mechanic but it does sound like a vacuum leak. Does the squeal change pitch if you rev the motor up? As for fitment. You would have to get one from that model. If you get me the full details of teh bus I could find a price for you on the vac booster. Maybe even a change over unit.
Maybe be stupid, but have you tried taking the master cylinder off and refitting it? Might be getting the squeal where the two meet.

[Cuppa]

Thanks. Haven’t tried revving the motor whilst the squeal is occurring, it is generally of very short duration as I immediately back off a tad & it stops. Hadn’t tried refitting it, I’ll have to lok at the manual again to refresh my memory, it’s a couple of year (but less than 3000kms) since I fitted it. I would be interested in pricing a replacement. Will send you a PM with bus details.

EDIT. Thinking about it further I don’t think it is likely to be a leak between the master cylinder & booster as if it was it wouldn’t only occur under severe braking. Have just had a look at the manual again & found that there is adjustment. There is both an ‘output rod’ (m/cyl side of booster) & ‘input rod’ (pedal side of booster) & both DO have adjustment after all. However the adjustment is minimal (max of 0.5mm on output rod) & getting it wrong can result in binding brakes, so I think I might need to find a brake specialist to try adjusting it for me. Trouble is the two brake specialists I spoke to in Ballarat assured me that adjustment was not possible, only replacement. Of course whether adjustment has any relationship to the squeal is an unknown.

[pearcey]

G`day Cuppa
If you loose all vacuum you will still have a good pedal but no assistance and the vehicle will be blxxdy hard to stop but you will still be able to stop.
boosters are not normally interchangeable so the correct one should be used
Your squeal is most probably coming from the valve which allows atmospheric pressure to rush in apply the desired pressure for the power assistance.
This valve is housed in the large Bakelite piston and is connected to the pedal. there is a foam air cleaner attached to the push rod under the dust boot which also acts as a muffler but over time disintegrates allowing some strange noises to escape. The booster should be a double diaphragm unit and one of the diaphragms may have a small leak under extreme braking (highly unlikely) as you would notice some change in the effort on the pedal and efficiency would drop away quite quickly.

[Robo]

Cuppa anyway you look at this air is passing past something and causing it to vibrate.
to me if this was to keep up sooner or later may get worse you'll lose what next ? brakes maybe?.
Gee wiz I don't want to be in that situation ever again, it's a heart stopper!!.

No brake or clutch expert, but if the foam is a muffler, it may not only deaden the noise but may also
help reduce the amount of vibration in turn reduce premature wear n tear.
given it will help slow air movement in/out.

I love these little brain teasers helps ya work on the fly when it matters.
like you Cuppa I like break down free holidays.