How much air pressure should you apply to bleeders like this?
ABSOLUTELY NONE. You apply pressurized BRAKE FLUID to those adapters, NOT AIR.
1. I get the sense that the original poster has never used a pressure bleeder before, and he thinks the master cylinder adapter IS the pressure bleeder.
You pressurize the TANK that holds a BULK QUANTITY of brake fluid. The pressure in the tank forces brake fluid through the adapter into the master cylinder. You do NOT pump air through the adapter into the master.
Some novices on "The Internet" think they can pressurize the master cylinder with air, using the fluid in the reservoir to bleed the brakes with. THIS DOES NOT WORK, and it's a great way to pump shitloads of air into the hydraulic system.
2. Those garden-sprayer bleeders are junk. The fluid is contaminated with humidity (water) before it even gets to the master cylinder. REAL pressure bleeders use a rubber diaphragm to separate the fluid from the pressurized air. Humidity in the pressurized air never touches the fluid.
3. A REAL pressure bleeder will accept a surprisingly high amount of pressure. I typically start with 10--15 psi, and I don't have problems until the pressure is less than 5 psi. Relieve the pressure at the master cylinder using the shut-off valve and then crack a bleeder open briefly, BEFORE removing the adapter, or it'll be messy.
https://www.amazon.com/Pressure-Bra...keywords=Brake+bleeder&qid=1571697700&sr=8-10
But be sure you're getting the adapters you're actually going to need. They've changed what's included with the pressure tank since I bought mine years ago.