Merc trim wont adjust up while boat is on plane

[wellcraft-classic210]

Gents
I have a 1983 Merc 260 Alpha one with what appears to be an oildyne pump -- ( Dipstick with flat head screw top used to check & fill reservoir. )

I inherited a problem from the previous owner in that the trim system does not have enough strength to raise the drive while the boat is on plane -- But works perfectly otherwise with no intermittent issues.

What I have so far is that ;

All the electronics works as expected and is very repeatable
The cylinders move freely when the hydraulic lines are disconnected. ( I can move the drive freely be hand up and down )
By rigging up a break line and high pressure gauge I verified 2600 PSI on the trim pump output fitting for cylinders extending / drive up and 600 PSI on down.
It also hold's pressure / trim position well under load if I raise the trim before applying the full engine load to the prop etc.
I did verify the cylinders are NOT pre apha which required a high pressure pump.

I now suspect something internal to the trim pump-- perhaps a fluid return issue or valve body issue internal to the pump. --Wondering if anyone has seen this issue first hand and or has advice in locating the root cause.

Also -- Should I look for a newer pump style if I do swap it out,


Thanks in advance !!!

 

[Scott Danforth]

dirt in a check valve would be my guess. take the pump to a hydraulics shop. they can rebuild for under $100

 

[wellcraft-classic210]

Ok Thanks for the advice !