OP - I've got some questions and some good news to share with you about my own car in hopes it can help you a bit.
Please remind us what the MC bore size is for your car. I'm also curious from one of your posts a few weeks ago why you were smelling burning brakes and saw smoke? What did you do to fix that issue? Lastly, can you please share a photo of your booster and let us know where you got it?
If I go back and read your original post #1, your brake symptoms describe my own from a few years back, except I have discs up front. I chased down this brake pedal issue for years, but I've finally got mine fixed and working well (just as of YESTERDAY!). Here's a summary of what I did to fix mine.
- I discovered I had a brake line leak at the bottom port on the brass distribution block going to the rear drums. I replaced the rear line and fitting at the distribution block.
- I installed new ceramic brake pads up front. Whether disc or drum, most of the braking power comes from the front brakes.
- I used to run a 15/16" bore Master Cylinder. The pedal travel would go almost all the way to the floor and was quite light feeling. That did not inspire confidence. I then learned that normally a 15/16" bore MC is more suited for a manual brake setup. I swapped in a 1-1/8" bore MC (Raybestos blue). Pedal travel now stops well above bottom of floor and pedal feel is not as light as before, but still fairly light. I would actually like it a bit firmer, but it's fine.
All of the above was completed over a year ago and did not completely fix my problem. Then recently....
- The BOOSTER - I fought and fought this in my mind thinking that a new booster is a new booster and how could a different booster really help me? It can. I was wrong. The booster I had installed when my troubles started was one of the gold ones like Randy posted above. It came with an all in one kit I from Master Power Brakes that used to be on the car. See below. There are currently no items remaining from that original Master Power Brakes kit.
When my car was at KernDog's house getting the A/C and Borgeson steering installed, I went ahead and ordered a new Bendix dual diaphragm 8" booster from Dr Diff and had KD install it. It looks like this:
View attachment 1677127
So all should be good right? Well things were MUCH improved. Pedal feel was no longer mushy and the brakes now grabbed with bite and not like before where I couldn't lock up the brakes at all. I could not believe a booster could have that much of an effect. Now before this booster swap, I had fixed the rear line leak. That actually made a big difference with the gold booster and did allow me to finally lock up the brakes with the gold booster, although not much bite from the brakes and the brake pedal still felt a bit too firm, requiring medium/high leg effort. After swapping in the black Bendix booster, the brake pedal actually felt TOO light, but not mushy at the bottom, required very little leg effort, AND the brakes were now VERY touchy. I was frustrated and immediately did not enjoy driving the car. I actually liked it better before the booster swap.
So...I decided to check the Master Cylinder push-rod gap between the bottom of the bore opening in the Master Cylinder and the Booster push rod on the engine side. The Bendix booster was installed with the push rod having adequate clearance to avoid brake dragging, but close enough to give good stopping power, but could pedal feel be better?
Last year I came across this H-shaped booster push rod adjustment tool. Many here say they've never used one and don't need it. Maybe not, but I bought one and wanted to try.
View attachment 1677140
As you can see, after unbolting the MC from the booster (no line disconnecting required), you place the short side against the face of the MC and seat the sliding metal rod to the bottom of the bore and then lock the lock screw. Then you place the long side against the booster face with the gauge positioned on the flats of the booster where the elongated oval indented area is (the gauge will only fit this way and requires an approximate 20-degree clocked rotation for correct placement). In front of that long side metal rod add a .020" feeler gauge and now unscrew the adjustable booster push rod until it contacts the feeler gauge. This provides a .020" gap between the booster push rod and the bottom of the MC bore. The H-shaped tool user instructions say the gap should be .005" minimum. Elsewhere I've read that an ideal target gap is .020". As a side note, I phoned Booster Dewy and he agreed with the .020" gap. (He also said he doesn't rebuild new boosters and thought my new light touchy pedal issue could be related to the push rod depth setting, so here we are.)
As found, I measured the booster push rod gap at around .050". I brought the push rod in closer by unscrewing the adjuster and setting with this H-gauge and a .020" feeler gauge. Bolted it all back together and took it for a drive. BINGO! All is good now. The touchy pedal is gone. The brake feel is still light, but not as ultra-light as before and initial pedal engagement up top is much more predictable. I can lock the brakes up at will and the fronts bite first (and with no proportioning valve since I have the smallest rear brake wheel cylinders). And remember, I'm still running a 1-1/8" bore MC and the pedal requires very light effort. If I went to a 7/8" bore MC, I know I wouldn't feel pedal engagement at all. I drove someone else's car with a 7/8" bore and it was scary because of how light it was and how low the pedal traveled towards the floor before the car stopped.
So in the end, my advice to you is fix all leaks (I think you have), install a new dual diaphragm Bendix style booster from Dr Diff and make sure you set the push-rod gap correctly to .020". I recall you saying .015" on yours which should also work, but are you sure it's .015"? How did you measure this? I'm wondering if you set it accurately to .020" and use a 1-1/8" bore Master Cylinder, maybe your pedal feel will improve?
Anyway, I'm pleased beyond belief with how my car stops now. It was an ongoing saga for me and now what a relief. I hope you get your car not only stopping ok, but stopping excellent with great pedal feel.
Thanks to
@Kern Dog for talking me into trying the Bendix booster and for installing it and thanks to
@1 Wild R/T for the feedback on the booster differences.
