A similar problem also recently occurred with my Alpine Series II which of
course had a non standard booster. I noticed that the brakes would come on
initially with no boost then with only a slight additional depression of the
pedal alot of boost would arrive and decceleration would be rapid, very
scarey on a road with signs warning of ice.
When I got to my destination I noticed that the revs went up and stayed up
when the brake pedal was only slightly depressed, extra depression brought
the revs back down. This indicates that the valve which switches from
atmosphere to manifold vacuum was stopping half way across (or at least not
sealing one side) thereby letting atmospheric air in while still sucking air
into the inlet manifold to try to increase the boost.
This is what I think:
If the revs stay high no matter how hard you depress the pedal then the
valve may never seal or there may be a leak in the diaphragm which
separates the two bodies of air. One side should always be at manifold
pressure the other being adjusted between manifold pressure (when the
brakes are not applied) and atmospheric pressure (when maximum boost is
required). If the valve is not working properly then it may be letting
air in to increase the boost but not shutting off the vacuum from the
manifold thus allowing a flow of air. A leak in the diaphragm would allow
the atmospheric pressure air to flow through to the constant vacuum side;
again giving a constant flow of air with the pedal depressed.
Any leak in the hose from the manifold would manifest itself as a constant
problem, not one which only appeared when the brakes are depressed because
the inside of this hose is always at the same pressure as the manifold.
Pedal pressure should compare well with modern vehicles, but may be slightly
higher, braking performance should be pretty good. From what you say your
booster is not working properly at least some of the time.
The bad news is that any booster problem that happens only when the brakes
are applied will almost definitely require having the booster dismantled.
My solution was easy, the booster wasn't supposed to be there so I took it
out, the pedal has to be pushed harder now but braking is far more
predictable and confidence inspiring.
Anthony, Alpine II, Australia.
>With interest I follow the servo discussion and I start wondering
>if my Alpine doesn't suffer from servo problems as well.
>My revs go up and stay up under braking then what exactly could be
>leaking (I've put a new hose between the servo and inlet manifold as well)
>And then the leak should be audible shouldn't it ?
>Also my brake performance is not very good at least if I compare it with a
>modern car with a servo.
>Even compared with my Imp without servo, I used to think it was just the
>weight of the car.
>So how can I check if my servo is working properly, should I be able to lock
>the wheels when braking hard like on my Sunbeam Imp Sport with sevo ?
>Another thing is that when comming to a halt at lower speeds it seems that the
>front brake pads lock/grip, so I have to reduce the pedal pressure !
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