Hey everyone,

I’m looking for some help with my drum/drum brakes on my ’65 F100. I recently installed a dual‑reservoir master cylinder with an 8" vacuum booster from Master Power Brakes (MPB). After converting from a manual single pot, the brakes are extremely touchy. During low-speed testing (3–5 mph), the truck stays straight, but moderate pedal pressure causes the rear drums to lock first with a chirp, while the front-end nose dives. Between test runs I verified and adjusted the pushrod clearance to with-in the MPB's recommended range, but this did not fix my issue. The brakes behave the same before and after adjusting the pushrod distance.

Since this is my first time working on drum brakes, I’m not sure if I’m dealing with a booster mismatch, shoe adjustment issue, or something else entirely. If anyone has experience with drum/drum setups using a modern booster and dual master, I’d appreciate any advice, things to check, or common issues to consider.

- Thanks

 

Most of the time a power brake setup doesn't go from pedal to pushrod to booster to master cylinder. In between the pushrod and the booster there is another linkage that reduces the amount of leverage that the pedal has over the system. You might have to drill another hole in the pedal that's down closer to the tread to get rid of some of the leverage. Or swap to a master cylinder with a larger bore.

 

You're going to need a proportioning valve on the rears. Did you adjust the pushrod between the booster and master?

 

No proportioning valve used with drum brakes the dual master drums did have a differential pressure valve but all that does is turn on a dash light if you lose brake pressure in the front or rear.
Providing your brakes are properly adjusted and bleed it almost sounds like that GM master cylinder doesn't have a residual pressure valve in both reservoirs or the front one isn't working. You should call the place you got all that from and ask some questions. Drum brakes require a residual pressure valve in the master, or they will retract too far away from the drum. It's a little late now but you could have bought a Ford booster and master for a 68-72 drum brake truck for a lot less money.

 

Power brakes need roughly a 4:1 pedal ratio, manual brakes are generally 6:1 ratio. My '66 C10 has manual brakes and the pedal ratio is 6.25:1. Adding a booster without changing the pedal ratio adds too much output force into the system for a given pedal input pressure, making the brakes touchy. Drums are already self energizing so they're somewhat "power assisted" already and it's easy to overboost them.

Check your pedal ratio, measure from the pivot point to the pad, and from the pivot point to the pushrod. The booster kit may have come with a bracket with built in linkage that alters the pedal ratio, but if not you'll need to drill a new hole in the 4:1 leverage location.

Double check that the master cylinder is drum brake specific and has built in residual valves, they are needed to keep a constant 10 psi on the wheel cylinder cup seals to keep them "inflated" on the return stroke, so air isn't sucked in which will make the pedal go soft.

No proportioning valve is needed on drum/drum, the staggered wheel cylinder diameters handles the front to rear brake bias.

If you haven't worked on drums before, triple check that they're assembled correctly and use a FSM or post pics here of how they're assembled so we can double check- don't just copy what you found inside, the last guy might not have put them back together correctly. Many applications have a short shoe and a long shoe and the short shoe needs to be at the front of the backing plate- I didn't know that the first time I worked on old truck drums. There also needs to be a chamfer on the leading edge of the shoe lining, use a grinder to add a 45* bevel if it has a 90* edge. The rear grabbing before the front could be an initial adjustment issue, or an assembly issue.

 

@theastronaut
This is a timely post for my own selfish needs so I thought I'd add some photos to be sure I'm reading this right. I also understand that this is for Drum/Drum setup. I have a '65 F100 that I'm assembling and obtained a small brake booster with master cylinder a few years ago (small in comparison to the one I salvaged from a '74 F250 for another project).

Question: what are the signs that the master cylinder is drum brake specific and has the valves you mentioned?
Additionally, I've taken the liberty to photograph my brake pedal marking the dimensions as I understand your description. I obtained the 6.1 ratio described, and solving for 4.1 I obtained about 3.4inches from the pivot point. This distance is VERY close to the bend in the pedal as shown below. Additionally, are there any alignment concerns with the pushrod at this new location?



overall view of brake pedal showing measurement locations


Measurement taken at approximate midpoint of pad location.

Brake pedal turned on side to show approximate bend which limits amount of distance available for re-locating the brake push-rod location.

 

There aren't any external indicators of a drum master cylinder, you'd have to disassemble it to find out if it has a built in residual valve. Or you can bench bleed it then use a pressure gauge to see if it holds around 10 psi when released.

I haven't done a power brake conversion on a F100, from what I see here it seems like the booster usually has a mounting braket with a built in linkage system that reduces leverage at the booster/master cylinder so no pedal/pushrod attachment modification is needed? If the angle the pushrod needs to be mounted at for a more typical 4:1 ratio won't work out then the brackets with leverage reduction would be the way to go. Or, a compromise of like a 5:1 ratio to get the pushrod to fit correctly if needed and make up the difference with the master cylinder bore diameter- smaller makes more pressure, bigger makes less pressure.


Another option to get more braking power is to alter the master cylinder bore. If you leave it manual brakes and go to a 1/16 or 1/8" smaller bore you get a pretty good amount more hydraulic pressure for the same input force, at the expense of more pedal travel. If you keep the drums adjusted (or the self adjusters work correctly) and there's no air in the system then that can work out just fine. My personal truck is disc/drum with a 6.25 pedal ratio, 1" bore manual master cylinder, 12" rotors, the typical GM D52 big single caliper, and stock rear drums. Pedal effort and response is great, it's easy to modulate at lock up and pedal travel is good too. We have a '58 Impala at the shop with a 6:1 ratio, D52 calipers, but smaller 11" rotors and stock rear drums. We used a smaller 7/8" bore manual master cylinder for more output pressure to make up for the smaller discs and slightly less leverage. Haven't test driven it yet, but that should get us in the ballpark.


I would suggest doing the math for the pedal ratio, input force, booster force (CPP has a chart), and MC bore size to see what your line pressure ends up at before making big changes. It's easy to overboost non-abs systems- you are the anti-lock mechanism so making the pedal overly easy just makes the brakes lock up very easily in a panic stop situation which isn't safe. It needs to take some effort to lock up the brakes since abs isn't there to save you. I always suggest occasionally doing panic stops on purpose somewhere it's safe to build up muscle memory so the first time you need to panic stop you'll know what to do more instinctively.



Here's a sample of the math on my own truck compared to the typical aftermarket power disc brake conversion. It's not as different as you'd think so I don't really get the need for adding power brakes in most street truck situations. This is a disc-oriented comparison but the principle is the same.

 

Well as others have stated, this is a timely series of posts in this thread, and thanks theastronaut for a very well articulated and informative post. I have been wanting to upgrade the brakes on my '66 CS to disk on the front and retain the original drum back axel. But I have been on the fence about adding power brakes. In principle it is appealing, but for various reasons I have remained on the fence, until now. Now, I think I will approach my brake conversion (assuming I can track down all the right parts) with a bit more confidence. Thanks again! This is just one more example of why this site is so valuable.

 

Good info above ^^

Power brakes were optional for the 1965 and 1966 F100 and F250 (4 x 2) trucks - they used a Bendix (2504335) single diaphragm 6.75" vacuum brake booster with the same single pot master cylinders. They cleared the driver's side valve cover better than the larger ones.





The ones they installed had no cantilever at all. Direct straight through push.



I have a vice jig to hold the front side of the booster and then I use a common booster wrench to separate the two halves.







It's a bit like pop goes the weasel. From here the diaphragm, seals and filter can be checked and cleaned or replaced.

On Master Power Brakes web site for the booster kit, I think you are talking about is this page? If so, they show a choice of 1" and 1 1/8" bore and I was wondering which one you went with?

1965 Ford F-100 Power Brake Conversion Kit - 8" Dual Diaphragm Booster | Master Power Brakes

 

I have done nearly the exact same thing to a 54 F100 and the problem is the high leverage pedal coupled to a power booster with front drum brakes which are somewhat self applying make for very touchy brakes.

I actually learned to like the extremely low pedal effort, it had the through the floor pedals and I used the brake like a accelerator pedal, kept my heel on the floor and ran the pedal with my toes.

A front disk conversion would solve the problem