You repacked the pump replaced the balls if not flat valves if you have this never replace them with balls .
The problem is you didn't completely rebuild the mechine .
You still have to take apart the the primeing knob .
Its a drift pin on the knob drive it out remove the knob and disassamble the primed valve clean it DRYED PAINT .
Replace the Orings you can get a kit or hardware store was WD 40

It sounds that the sprayer is losing prime, after having primed OK. From this I assume that you mean that it starts racing and not building pressure. There are 2 possible reasons, the obvious one has been explained by others that there's a problem with one of the valves, the other is that it is sucking air through the inlet hose swivel. There's an O-ring on the end of the inlet hose # 704-121 that could be defective, to access it, remove the retaining clip by pressing the two ears together and pull out the elbow on the end of the suction hose.

I just wanted to kind of stress how important it is to really clean an airless with what ever the solvent is for the kind of paint your running thru the spray rig water for latex and mineral spirits (paint thinner) for Alkd base paints-
Here is the best routine for cleaning an Airless spray rig-
say were using Latex-
put sump hose in a 5 gal bucket wipe off the hose with a rag and leave it in the water and make sure you clean the screen or filter at the end of the sump hose (delivery tube)
take the tip off turn rig on and chase the paint out of the line back into paint container when it starts to pop and you hear bubling it is at the water now run it into another 5 for a minute the open prime valve to get paint out of the manifold due this for a minute

Now remove the filter from the filter housing leave it out but in water
now run clean water thru the hose and gun then open prime valve and run water thru the dump hose

so repeat this till it all comes clean don't forget to put tip on the gun and blast water thru it I use a nylon brush on the gun and body and keep it all clean and looking good

Now here is how I store my Airless pumps and is a little different
if I am gonna use the sprayer in the next week or so I will use the Coro Check or any other pump preserver which is basically liquid Teflon I also soak the gun in it for a few minutes NEVER LET YR PUMP GET COLD ENOUGH TO FREEZE.
DON'T LEAVE YOU SPRAYER TORQUED WITH FULL PRESSURE IN IT LET IT OFF NOT QUITE ALL THE WAY

Now if I am not gonna use one of my sprayers for awhile or if its very cold out I make a solution of clean new paint thinner and motor oil like about 25% - 30% oil to thinner ratio this keeps everything nice and lubed up cuts down on the rust @ check balls and most importantly the "leather washers" in the pump that wipe off the rod.cover it up to keep dust off the electric motor and the brushes.

Thanks for reading,
Perry @ Tradesman Painting & Decorating Lic.778435

These machines aren't made to sit around that's why pretty common another way that works for me is use gun wash first clean it all out and flush with water before use it will loosen everything up and keep all the pressure off the system when storing

Sounds like you havent got the ball bearing in the prime valve seating properly did u wash them thoroughly befor setting them in if you rebuilt yourself not a good idea will cost you arounf 400 ponds for a complete service with new piston and valves