I wasn’t happy with the performance of the brakes, so I actually ended up pulling a lot of the components over the last few weeks to re-check everything. The two main problems were the rear brakes locking first and excessive pedal travel.
The problem with the rear brakes locking first ended up being the pressure differential valve in the distribution block not being centered. This also caused the brake warning light to come on. I didn’t realize that when bleeding the brakes you’re actually supposed to use a tool to keep the shuttle valve centered, otherwise the pressure differential you create will push it towards either the front or rear circuit. Once I figured out it had tripped towards the front circuit, I removed a rear brake line from the dist. block and slowly tapped the brake pedal until the brake warning light went out. Afterwards I immediately pulled the switch and installed this tool in it’s place: https://www.npdlink.com/product/tool-brake-pressure-differential-switch/178818. In addition to triggering the brake warning light, if the pressure differential valve moves towards the front brake circuit (detecting a front brake failure) it will cause the rear brake circuit to bypass the proportioning valve. The idea being if you lose your front brakes you want to have maximum brake pressure to the rear circuit. This was the cause of the rear brakes locking first. Once I got the shuttle valve centered and brakes re-bled the fronts would lock up first as expected.
To tackle the excessive pedal travel first thing I looked at was the pushrod depth on the brake booster. Initially I had checked it with a tape measure, but didn’t realize you need much more accuracy than that. I ended up using a brake pushrod depth tool to set the depth. Pulling the master cylinder to do this was a chore. I don’t know if it’s the combination of the Bendix-style booster and MC on a 67-68, but the master cylinder is right up against the shock tower. I tried every which way to remove it, but I didn’t want to mangle the shock tower or MC bore so I ended up loosening the booster to allow the master cylinder to come out. I set the depth to .004" clearance with the tool and a feeler gauge.
I put everything back together and bled the brakes again. I ended up buying speed bleeders this time, they work great and save a ton of time over any other method I’ve used. The pedal was better, but still didn’t have a good feel. I decided to take a look at the rear brakes and found that the drivers side shoes were gone and the wheel cylinder had started leaking.
I ended up just installing new shoes/drums/wheel cylinder/hardware and parking brake cables on the rear. The spring kit I got was not great, had to end up reusing a couple of the original springs but it all went together OK. Once I got the rear shoes adjusted, parking brake cables snug and did the final on-road adjustment using the reversing method the pedal finally got decent feel back in it. In my mind it should feel a smidge firmer, but it’s way better than it was before and feels safe and comfortable to drive now.
One note about the parking brake cables, 68 Mustang and Cougar cables are the same parts. The Cougar uses a longer cable connector than Mustang to account for the longer wheelbase (The Cougar connector is about 2" longer than the Mustang part). The cables I bought (Inline Tube) were manufactured too long, and there was not enough travel on the adjuster to get them snug. I ended up having to use the Mustang-style cable connector to shorten the entire assembly and bring the length of the cables back into the effective range of the cable adjuster. I don’t know if the ACP or Scott Drake cables are any better.
While I was battling with all this, I ended up getting a couple brake pressure gauges so I could see the actual pressures being produced from the front and rear. Once everything was working correctly a panic stop level of pedal application produced 1100psi front/900psi rear with the engine idling. Enough pressure to stop the car adequately, and the proper front/fear balance to prevent rear lock-up. So in the end I spent a lot on brake-specific tools I’ll probably use very rarely if ever, but worth it for me to have the piece of mind that the brake system is setup and working as it should.
