oil pressure

Suddenly lost oil pressure on my allis h3. New rebuild only started about  6 times. Up till now had good 30 plus psi. I read on internet they lose prime, Is that common?  I always think the worst but  they said to just reprime pump?

It did not lose pressure while running. Lost pressure between startups.

Live and learn. No I did not rebuild oil pump. Looked good to me. At seventy six years old and lots of cars, trucks, tractors and dozers I've seen pumps slowly lose pressure but never lose prime just sitting there. I guess I'll just do like I did as a kid. Before I use it just Prime the Pump.

DrAllis wrote: Probably all the other engines you've worked on didn't have the oil pump 12 inches above the oil level either !!

Indeed!
When I overhauled my first Model C some 35+ years ago I was perplexed by its rather bizarre oil system; this was before the days of internet and I was unable to source new parts so I just cleaned it up and it ran fine, never gave any trouble.

My 1941 John Deere Model A, on the other hand, has its oil pump submerged at the bottom of the sump where it is impossible to lose prime so long as there is oil; I have had water condensation settle down there and freeze, causing the oil pump drive to strip out

I am most circumspect about draining any condensation out of it after that little learning episode.

I got the pump reprimed by taking oil line lose from filter (supply side) and slowly filling line with oil till full.  Since then it  has lost prime again. Always when I shut it off never when running.  I guess I'll drain oil and check the sump and pick up tube again but I went over it real good before?

These pumps don't make any vacuum for pulling the oil from the sump after a drain interval? Sounds like the pump body and gears are worn severely to me. There should be a specification in the manual for this. Clearance from gear tip to housing, gear backlash, scoring of parts, etc. all should be checked at rebuild or refurbish time. I'd wager these are way past tolerance limit if losing prime. Some pumps utilize a ball check and spring also. I don't know this application at all but something was missed at rebuild I'm thinking. Not being critical at all but this needs checked really well. 

Thanks Les; I would not have suspected a vane type pump for engine oil at all. I've never been into, or worked on one of these myself. Vane pumps inherently draw more vacuum during rotation than gear type pumps so there is a definite problem rearing it's ugly head here. Wonder if it has anything to do with the filter not being original? Seems a bypass type is used rather than full flow from an earlier post?