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I've got a very old MeiDai Top floor pump, like this picture ...

enter image description here

It suddenly stopped working, no resistance whatsoever from the plunger. I'd like to repair it.

So far, I bought this "741" washer (https://www.amazon.com/gp/product/B009VU2D64/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1) and installed it. Only hassle was, the hole in the middle is considerably smaller than the plunger on the pump. So I reamed it out a little with a 3/8" drill, which didn't really enlarge it much. But I was able to stretch it onto the end of the thing, and then rotate it so that it screwed onto the threads like a nut would. And the thing seems to seal and move air.

But, evidently the check-valve is broken or stuck - when I connect to a tire, air pushes the plunger up. To access the check-valve, I removed the barrel/cylinder from the base (big cast-steel piece into which the pressure gauge also screws), by putting the base in a bench vise, gripping the cylinder with a chain-type vise grip, and unscrewing; the chain put a little dent near the bottom, but hopefully that won't hurt anything. But I don't see any signs of a check valve. Just a channel between where the cylinder screws in and where the pressure gauge screws in, and another channel from there to where the hose screws in. So I'm perplexed. There's also a funky little rubber piece that sits underneath the pressure gauge where it screws into the base, but I don't see how that could be the valve.

Here is a picture of the base, and of the little rubber piece. I seek advice on where the check valve might be, what happened to it, where to get a new one ...

Is the check valve in the hose, where it screws onto the pump ?!?

enter image description here

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That little rubber piece is indeed the check valve. The protrusion (on one side) fits down into the hole in the base (where the pressure gauge screws in) and prevents reverse flow.  I cleaned up the corrosion in the base, and it works great !

Failing that, I could have simply spliced a check-valve into the hose. I'm advised that the thread in the base "is an unusual thread, Japanese pipe thread", so find a check-valve to screw into the base probably would've been a non-starter.   \

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    To avoid messing up the plunger washer on the dent I put near the bottom of the cylinder, I simply put a tiny-size hose clamp on the plunger shaft between the handle and the spring, to prevent the washer from being about to go down quite as far to where the dent is.
    – RustyShackleford
    Oct 21, 2021 at 23:09
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    If you put a check valve in the hose the pressure gauge wouldn't tell you the pressure in the tire any more, it would be the pressure in the pump.
    – Noah Sutherland
    Oct 21, 2021 at 23:51

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