Mercruiser Prestolite pump question.

[Alter Bootsmann]

I have a '73 model Mercruiser 888 with a Prestolite tilt trim pump. Pump motor runs fine except for one thing. When I trim out at speed my trim cylinders will not maintain sufficient pressure and leak down within minutes or less.
No oil leaking anywhere. I have replaced lines when engine was pulled two years ago. Is there an internal pump issue going on here? Control valve or something?
I know replacement pumps are high and if this cannot be fixed it may just pay to buy a used pump assy.
Any helpful comments are appreciated.

 

[old islander]

Re: Mercruiser Prestolite pump question.

Most likely check valves in the pump. Replace it.