460 diesel Brake servo/booster
My brake servo/booster runs out of boost after 2 quick applications of the brakes.
The mechanical vacuum pump is working fine, monitoring the vacuum on the pump side shows there is no leak.
It just seems to get emptied by 2 brake applications.
It is worse if the rear brakes are adjusted as tight as possible.
Any ideas ?
Robert
1985 300GD LWB
I remember the system of my 460 300 GD to be very poor in this respect but it would run out of servo after two deep aplications of the brake at standstill and with the engine at idle. At operational speeds you would not notice a thing, because the system would create sufficiente vacuum in the meantime.
I was always told that this was normal and due to the lack of sufficient vacuum storage, matter that was corrected in the 463, which uses the first member of the chassis as vacuum reservoir.
It is correct that the 460's have no vacuum reservoir and hence are not as "good" in this respect as a 463. However, with age, the internal rubber diaphragm in the booster fails and causes a leak in vacuum. New units are very expensive, but a s/h unit will be a gamble.
I have had this problem on a couple of 300GD's and a new servo was the cure.
However, check the vaccuum on the booster side of the one-way valve and other fittings first.
My 300 GD brakes are top class so it ain't a design fault. There is obviously a fault in your system.
Not sure what you mean by "pump-side", but to see if there is a leak in the internal diaphragm, inside the servo drum, you need to check whether it maintains a vacuum between the servo booster and the one-way vavle, which is before the pump.
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Arnie
The vacuum doesnt drop on the pump side of the one way valve, I assume that if there was a leak anywhere it would show up here.
Its just that there is little capacity in the resevoir and it disappears after 2 applications of the brakes.
As adjusting the rear brakes as close as possible reduces the movement of the brake pedal and seems to improve the servo/booster capacity it is as Jesus says just a design issue.
Robert