Master Power Brakes Brake System Diagnostic Kit - Brakes 101

Talking over my brake problems with the guys at Classic Performance Products revealed that they sell a double diaphragm booster in 7-inch diameter, so a quick trip to their shop saw me leave with what I hoped would cure the problem. While I had the master cylinder removed from the booster, but not disconnected from the brake lines, I used the depth gauge from the MPB kit to check the cylinder and booster. All was good, with 0.020-inch clearance between the pushrod and the cylinder piston. If this is adjusted incorrectly it may cause the brakes to drag if too tight, or give too much free-play at the pedal if the gap is too large. While the master cylinder was off, I also measured the bore diameter, and came back from CPP with a slightly smaller one in addition to the booster, dropping from 1-inch to 15/16-inch.

Here's the old (right) and the new (left). Both share the same mounting bolt pattern but the double diaphragm version is a littler longer. While this wouldn't be a problem in many cars, it will cause interference issues in my pickup. Temporarily I unbolted the inner fender at the bottom for clearance.

With the double diaphragm booster fitted, and with the vacuum back at 17 inches with the motor at idle, the rear brakes showed almost 700 psi. A huge improvement! This compares well with Classic Performance Products' own specs which state that a dual diaphragm 7-inch booster with 23 inches of vacuum will provide 175 psi of 'free pressure' (the pressure the booster can provide, as opposed to 'manual pressure', that exerted manually on the pedal without the booster) with 100 lbs weight applied to a pedal with a 7:1 ratio and a 1-inch bore master cylinder.

The final step in my quest for better brakes was to swap out the 1-inch bore master cylinder for the slightly smaller bore cylinder from Classic Performance Products, reasoning that the smaller bore might give more pressure at the expense of a little further pedal travel. CPP's cylinder is supplied with four bungs to seal the outlet ports during bench bleeding, as well as a rubber boot and bore adapter. Manual brake applications require the pushrod extend a couple inches into the master cylinder, in order to prevent it from accidentally falling out. The adapter is therefore needed in power brake 'shallow bore' applications.

With the master cylinder clamped in a bench vise, the bungs were tightened in the outlet ports.

CPP gave me this small syringe with which to fill the bore of the master cylinder with brake fluid without trapping any air. Be sure not to mix silicone and regular brake fluid if you're working on an existing system.

Using a rod to act as the booster pushrod, the master cylinder was slowly operated to release any air bubbles.

The bungs were next removed individually and the large syringe from the kit used to 'back-bleed' the master cylinder through the outlet ports. A surprising amount of air was released back into the reservoir during this stage. The bungs were then refitted in order to install the master cylinder on the car.

The master cylinder was next fitted to the booster, after checking the bore depth again. With permanent bungs in the left side outlet ports, the brake pipes were fitted to the right side one by one, each one left slightly loose and the pedal gently applied to bleed air. The whole system should be bled, but this sometimes can work, and did in this case.

With the booster and master cylinder fitted, I again checked the pressure at each wheel, the rears still showing 650 psi and the fronts up to 750 psi. Master Power Brakes recommends they should be 800 psi for disc brakes and 500 psi for rear drums, so the pickup's a lot closer than when I started, but not quite ideal. However, the brakes work much better now, and feel like those on a modern car.