Check the tip switch in your master cylinder, really!
In the recent Treadle-Vac rebuild I was able to analyze why people would complain about Mark II brakes when it's really a maintenance problem.
The master cylinder piston is pushed back into position by hydraulic pressure generated by 8 springs returning the shoes to the relaxed position. That pushes the brake fluid back into the chamber, displacing the piston. It's aided by the coil spring in the brake booster. Both types of springs are necessary to return the ram into its resting position. If the ram isn't moved back into position the tip switch will never activate, letting fluid from the reserve replenish the brake chamber. When you apply your brakes the tip switch is supposed to close, sealing off to allow all of your foot pressure to push fluid out to the brakes, not leak back into the reserve. That alone would contribute to weak brakes. They are problematic.
The problem we all likely encounter is that the ram in the booster cannot fully return to its fully retracted position due to a ring of rust and gunk that is simply blocking the way of the leather seal, thus the vacuum shuttle, from fully retracting. Many of us have added springs to our brake pedal to help retract the ram, but, at some point a spring is not enough.
It's really quite simple to find the problem, but you have to remove the Treadle-Vac assembly. The original lubrication and seal sweller was Neatsfoot oil, made from cow hooves. It has evaporated. It was a leather treatment that swells and conditions leather. Without it you get the equivalent of a bloody glove that shrinks and no longer fits. That's the cause of poor brakes. Without a good seal you're just sucking air. If your rpm stumble when you apply the brakes it's because your vacuum chamber seal is causing a massive vacuum leak, affecting the fuel/air mixture. If you're sitting at a light with your foot on the brake it's most noticeable. You can verify the problem with your foot.
The only way to tell is to do a physical inspection. There are no oiling ports. Removing and checking the T-V is within your skill set.
1. Whatever you're doing on a Mark II disconnect the battery, chock the wheels, and set the parking brake.
2. Remove the shoulder bolt that holds the T-V shaft to the brake pedal. That will allow it to be pulled through the firewall with the T-V.
3. Remove the two large vacuum lines that run from the engine to the T-V and from the T-V to the reserve chamber built into the chassis on the passenger side. This chamber holds enough vacuum to have power brakes for a few moments. The device in the side of the T-V is the vacuum check valve/diverter. Under normal conditions the chamber is charged by the engine generated vacuum. If the engine stalls, or you open the throttle, the valve closes and diverts the stored vacuum to the booster and seals off the engine until vacuum is restored. The hose fitting facing sideways goes to the engine vacuum port on the manifold and the nipple that points down goes to the vacuum reserve in the frame. be sure you have actual vacuum line. You shouldn't be able to crush it closed. if you can someone has substituted water or fuel line meant to contain pressure.
4. Remove the two wires leading to the brake light switch.
5. Remove the brake line by holding the brass rectangle the brass nut screws into. Remove the nut counter clockwise facing the back of the car. Keep track of the copper washers.
6. Take the T-V off the firewall by removing 4 bolts with 1/2" heads.
7. When new they had clamps keeping the back cover on. Most broke off. Remove turnbuckle end and slide rubber and steel covers off over shaft.
8. Remove vent cover as the screw interferes with the shuttle travel.
9. Remove the vacuum line from the check valve and remove two screws to extract it.
10 Easy steps. With the back cover off and out of the way, check the three threaded holes for burrs, and then pull the whole shuttle out of the vacuum chamber.
Inspect the leather seal, noting that behind it is a special rope designed to wick Neatsfoot oil all the way around the seal. It needs to be soaked. Inspect the bore. It was originally zinc plated. You'll likely find rust pits in the bottom, where moisture can separate and eat the steel through the thin layer of zinc. Leaving untreated rust is the death of a new seal like a death of 1000 cuts. The bore of the T-V should also be lubricated with Neatsfoot oil to aid in the shuttle making a smooth advance and retraction. You will likely see a hardened line of lubricant that has trapped condensation from dew that forms anywhere there is steel. This needs to be smoothed, as this this what's hanging up the leather seal, preventing its full retraction that activates the switch in the reserve. It all has to work right for it to work at all. This may seem redundant, but you need to make them as-new to make them work at all.
Truthfully, how many of you have lubricated your Treadle-Vac? How many have ever purchased Neatsfoot oil? How many had even heard of it? I've learned its the base for most high end saddle and car leather treatments.
You can do this with a very basic socket set and a few wrenches.
