Best procedure for deteriming blockage?

[Dave Zeryck]

I'm new to a Cat27 ('76) with a holding tank installed. Was testing the loo this weekend after reading all the HM archives.The head needs lube, which I'll do. It'll pull water in OK, but won't pump into the holding tank. Tested with just water, it will pump out the thru-hull if the Y-valve is in that position.With y-valve going towards tank (bladder type), won't pump out, I feel backpressure.Since the bladder does not seem to 'inflate', I assume there is blockage between the y-valve

 

[Peggie Hall/Head Mistress]

If the toilet works in one direction,

the problem can't be the toilet, nor can there be a blockage between the toilet and the y-valve. If the tank isn't "inflated," it's prob'ly not a blocked tank vent either...although I'd check it anyway 'cuz switching to overboard may be relieving the pressure. If the vent is blocked, opening the deck pumpout fitting will relieve it (ONLY relieve it, not cure it...it's just the easiest way to find out)...so hang onto the cap when you loosen it. If there's no outrush of air, the only thing left is a blockage, which has to be between the y-valve and the tank...and I don't know any way to find it except to start removing hose clamps and eyeballing hoses. The first place I'd look is the y-valve...make sure the handle is actually turning it all the way around and not just enough to hang it up.