TBM thanks for the heads up, it is many a year since I had a mechanical pump in bits to replace it`s internals, though I think there may have been an inbuilt means of pressure regulation in the pump, i think that once pressure was at the preset level the diaphragm may have acted on the lever to hold it off the cam, and thus inhibit the pumping action till the pressure dropped.
In the old SU electric pumps pressure regulation was such that at the pre-set max pressure the electrical contacts were not allowed to close until the pressure was at or below the required setting. Those with similar electric pumps will hear the pump ticking rapidly when they first turn on the ign after lay-up until the fuel that has evaporated out of the float bowl is made up and once the correct fuel level has been achieved determined by the float rising to the extent it forces the needle valve to close and the pressure in the line to then rise to the extent that the pump stops ticking, until the engine is fired up and ticking like a heartbeat resumes.... Well at least that is how I remember it...I think.. More than happy to be corrected.
