Start with the simple cheap stuff.
Check the pump and pump to booster hose for cracks. Best to just replace that hose anyway. But for now run your paw over it it see if it has a rip.
Onto the booster. You can do a leak down test with a gauge. Simply install a vacume gauge on the pump line from pump to booster activate the pump then let it shut off. The gauge should hold steady vacume if not the booster or pump have a leak.
Into the master. Fill the reservoir and with someone else pushing the pedal slowly as the truck is running. Watch the fluid level in the front and rear. They should both remain level as the brakes are pressed. If one drops further then the other then a leak or master problem is suspect.
But before you go there we need to look at the proportion valve. Ok these can get stuck during bleeding. Sometimes there is a sliding part in there that allows so much to the front and so much to the rear. This sliding part can get slid one way then you need to go back and adjust it to center. If you reverse bled that can also mess them up.
Watch some youtube videos on the above and try the things out before buying parts.