ANSWER: The majority of the time, this is from the side seal (the little steel thingy that rides on the mix chamber - known as assembly18C) not sealing on the chamber surface.
Check your gun PRIOR to connecting the fluid manifold to the gun. Take your gun and hook up only the air hose and turn the air valve on to allow air to flow to the gun. If a check valve (assembly # 26) pops out, you have a problem with that side cartridge (assembly # 18.)
If the "A" check pops out, replace the "A" cartridge assembly or if the "B" check pops out, replace the "B" side cartridge.The check pops out due to air bypassing the side seal/chamber surface, flowing through the cartridge and into the check valve port and this causes the check valve to pop out.
Checking the gun with air supply only will save you a fortune in time, seals and frustration.You can also try flipping the "A" and "B" cartridges if you have not run any fluid through them. Switch the "A" and "B" sides. If the problem is in the side seal, the problem will move to the other side of the gun.
If the problem remains on the same side after switching/replacing the side cartridges, you could have issues with the front end body (assembly #11) being worn out or scratched.