I've rebuilt the Lavac pump in Boracay a couple of times now. It's not too difficult but it's also not for the squeamish.
Contact Lavac customer service
if you want to do this correctly. I always do things the wrong way.
I usually pump lots of clean fresh water (with some disinfectant of choice) through first then place a bucket under the cover plate. Then I remove the cover plate.
As the valves have usually failed by the time I do this what little water that remains has hopefully drained into the bowl. Then I remove the hoses and unbolt the pump, placing it in a bucket.
If the pump has been used for a while in salt water
there is a chance that "calcium deposits" have built up. I fill the bucket most of the way with water then add some phosphoric acid (also known as rust converter caution - poisonous and stings like hell in any cuts so use gloves).
After it has soaked for a while I scrape off any remaining "calcium".
It sounds like your diaphragm has failed so you'll need a repair kit. From my rapidly failing memory it's the same one as used in the Henderson Mk V but do check with your friendly chandler/Whale Pumps.
Follow the instructions in the repair kit carefully, particularly the bit about which way round the plates that hold the diaphragm in place go.
I found some parts
to be a tight fit so some care is necessary.
The only difficulty I had is that the replacement small screws in the kit were just a fraction shorter than the original ones. Using the original screws to put in the new joker valve caused a small leak which seemed to go away when the new screws were used.
Then replace the pump.