If you have a check valve in your return line Id sure be pinching off your return line and trying to see if it runs then.

I bought a Volvo yrs ago that the owner had put $600/700 of OEM parts under the hood to get it to start. The fault was a bad check valve that a quarter at a local junk yard solved.

The check valve was releasing pressure almost immediately and the system would never rise to the required starting pressure

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."