2006 GMC Yukon Brake Booster

You need to secure the mounting flange of the Brake Master Cylinder in a bench vise, remove the master cylinder reservoir cap and diaphragm, install suitable fittings to the master cylinder ports, install transparent hoses to the fittings installed to the master cylinder ports, route the hoses into the master cylinder reservoir, fill the master cylinder reservoir to at least the half-way point, ensure that the ends of the transparent hoses running into the master cylinder reservoir are fully submerged in the brake fluid, depress and release the primary piston until fluid flows freely from the ports with no evidence of air bubbles, remove the transparent hoses, install the master cylinder reservoir cap and diaphragm, remove the fittings with the transparent hoses and the master cylinder from the vise.

You should not allow the vehicle to be driven until it is diagnosed and repaired.

You need to remove the combination valve electrical connector, the front, and rear brake pipes, and the combination valve from the vehicle.