pishta
I know I'm right....
We all got one, or should get one. These are indespensible when it comes time to pull a vacuum on a distributor, EGR valve or even testing a new intake manifold install for leaks. Unlike your tongue holding the vacuum while you are sucking on the vacuum tube (yeah, we have all done that...) This guy tells you how much you are pulling and can do the leakdown test while you watch the guage, or even measure vacuum on a source. What happens when it fails to hold a vacuum with your finger over the hose: Cracks, umbrella check valve or O-ring.
If you have an older thermoplastic one like I do (cant even find a pic of it on the website) check for cracks around the guage boss.
Mine developed a nasty crack when I was screwing the guage back on after some pipe dope repair on the possible leaking threads. Turned out not to be that at all but if you do unscrew the guage for any reason, use some common sense and dont try and torque the guage back on pipe tight (like me, duh!) it has a NPT fitting and naturally, the more you tighten it, the more it wants to expand the boss and eventually, it will split. OK, so next time ill put a worm clamp on the boss when I tighten but in the meantime how do you repair that crack? Model glue....It works great! get your clamp on and apply some glue all along the crack (and any other crack you can see, mine is riddled with hairline cracks but none other seem to effect the tool) now seal off all vacuum ports with your fingertips and apply vacuum so it draws the glue into the crack. Release the pressure and reapply the glue so there is a nice light even bead over the crack and lay it up for the night. In the morning, Itll be nice and smooth, sealed permanently.
If it still doesnt hold a vacuum, the problem is either a bad O-ring (unlikely but EZ fix) or it is the umbrella check valve in the vacuum cylinder. That is where mine was bad and its an EZ fix too. Pop handle off fulcrum and **** to side just enough to access retainer plate screw,
unscrew and turn retainer plate counterclockwise so it clears lugs and pulls off. Pull the piston out slowly and youll see a Holley orange umbrella check valve at the end of the cylinder! Pull that out with your small fingers or some needle nose pliers and clean the back side of it along with the mounting flange. Mine had 5 years of grit holding it off the seat causing it to bleed off drawn vacuum. When you install the umbrella valve youll need to have access to the back of the cylinder. On mine it was just 2 screws that held the front part on. Remove them and you can get to the O-ring and the tit of the check valve. Pull it through until the swedge pops through. Now you can grease/replace the o-ring and reassemble the front. You can now push the piston back in, lock plate onto lugs and retainer screw. Check the one way vacuum valve on top for any other BS
Get it all back together and see if it holds a vacuum now, it should and you'll be back in business.
Note: Newer Mityvacs pull 2X the vacuum and are of a better design and Craftsman type models are all metal but are not capable of supplying a light pressure out the top vent port as the mityvacs can. HF sells a knockoff like this.
