T/T check valve

[newbie600rr]

I have an 82 50hp with the external twin cylinder tilt/ trim unit. It operates up and down without a problem but will not stay in the up position. It slowly leaks down. Both cylinders were just resealed. Reading thru past posts the check valve seems to be the problem. After some more playing with the tilt/trim unit I noticed that only one cylinder was operating. Would a bad check valve cause this or is there another problem? Should the motor stay in the up position under the power of one cylinder? Thanks for any info.