If you haven't bought the re-build kit for it, if you can find one, I would recommend an alternative instead. The Headmate was discontinued several years ago, and parts are getting scarce for it. Instead of investing in an obsolete head, buy a Raritan PHC-LBA or PHII-LBA instead of rebuilding the Headmate. This will effectively give you a brand new Raritan PHC or PHII, depending on which kit you buy, for a good deal less money. SBO, sells them right
here.
Peggie Hall has recommended the Raritan PHC/PHII head as one of the most robust, affordable, manual heads currently available.
BTW, I am in the process of doing this on my boat, as the WC HM90 failed at the end of last season, and I saw no point in repairing it, given the price differential between the rebuild kit and the Raritan PHC-LBA kit.
I'd point out that while you're at it, it might be wise to replace all the hoses and clamps. The 1-1/2 hose to get is Trident 101/102. The intake hose should be Trident 148.