Quote:
Originally Posted by Mark Misrule
when I run the engine, the pressure for the air brakes builds to about 4 bar nice and quickly, and then starts to drop off. also the air is not reaching the tanks further back under the vehicle.
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A 'protection valve' operates by limiting leakage (by 'protecting' the other circuits (reservoirs) to preset limits.
It sounds as if (maybe) the pipework supplying the air-doors is leaking and the normal supply to the service reservoirs isn't happening as the protection valve is preventing 'leakage'.
Normally, the air circuits would be arranged such that ancillaries (such as air doors) wouldn't be supplied until the service reservoirs were charged to at least the protection pressure.
In a working system, there should be enough air feeding the reservoirs to provide at least secondary performance before any air is 'leaked' to the ancilliaries.
Quote:
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A protection valve has the job of protecting the individual circuits from each other. They’re used to supply the independent circuits with compressed air. In the event of any circuit failing, that circuit is switched off automatically, the others continuing to be supplied with compressed air up to the opening pressures in the defective circuit.
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Look for an airleak in other than the service-brake reservoirs (possibly an unprotected add-on or modification of the piping).
Of course you might have failure of
both air reservoirs (such as missing connections or holed reservoirs) but it seems unlikely.
You might have to try disconnecting pipes to see which ones don't have air coming out and which one is leaking away the air after the protection valve has opened (at 4bar).