Barry Wolk
06-17-2025, 06:41 AM
Mark IIs built in the calendar year 1955 were sleeve valve units and post-1955 were poppet valve. My quest to understand the technical differences began with a gift of an unused poppet-valve from our resident mechanical genius "Mad Scientist", taken from us too young. He had a brand new Treadle-Vac that sat on his shelf for 50 years. His parents bought a new '56 model Mark II, one of the 300, or so, built as Introductory Units, a big batch of Mark IIs built specifically to sit in showrooms across the country as a means to familiarize the public with the upcoming model. They were put there on consignment and could not be sold until the product pipeline was filled. At that time the dealers were sent invoices so the cars could be sold.
I was told that his parents had a brake problem, but he had already fixed it by the time the new T-V arrived, hence it sitting for 50 years. Early on there were few spare part available. I asked why the unit was plated, instead of painted, and he had no good answer in his mind. It wasn't until I received the first day of production Mark II photos that revealed that all of the early cars had plated vacuum canisters. The unit I have has a soft zinc coating that was easily scratched in transit to the assembly plant. The first time they got wet they started to rust. Once discovering this flaw they were all painted black. I have confirmed with a major rebuilder that the vacuum chamber was plated inside and out. More about that, later.
My car was fitted with a sleeve-valve unit when I bought it. I believe that the two types of units work great when new, but the sleeve-valve unit started deteriorating the first time it was used. The only difference between the two units are in the means by which you activate the vacuum switch with foot pressure when you depress the pedal about 1/4" What they didn't know is that each time you press the pedal the soft bronze bushing slotted to allow vacuum into the chamber starts to wear as the bronze bushing is designed to be sacrificial. Each time the shuttle that moves turns on the vacuum that draws it into the vacuum chamber that movement under vacuum pushes the displacement rod into the master cylinder, closing the compensating valve, and pushes brake fluid out to the wheel cylinders. When they discovered that each bit of wear created a bigger and bigger vacuum leak each time you pressed the pedal. The change in effectiveness is so subtle that you simply respond with more and more pressure until you unknowingly have no power brakes at all. I believe this is the cause of the perception that the Mark II has bad brakes.
Instead of relying on a near friction fit the poppet valve unit uses rubber seals that positively traps vacuum in the chamber providing stopping power giving the impression that the brakes are "grabby", by comparison to a worn sleeve valve unit. I built two poppet-valve units, one from Mad and one that was a rebuildable core. I have a third vacuum chamber, but it is too badly pitted to be reused as the 6" diameter seal must have a smooth surface to seal against and slide on lubricated by Neetsfoot Oil, primarily used in high end leather treatments, meant to swell and lube the leather seal surface.
I contacted the remanufacturer to see if he could rebuild the sleeve valve and he said no. He said all of the units done by his favorite machinist didn't hold sufficient vacuum. I really wanted to rebuild mine, but no parts are available. In talking to my local old-school machinist and my CNC shop with a Rainman-level programmer they explained that there were actually two procedures needed. The first is to push the old bushing out of the aluminum housing. The hole needs to be checked for roundness. If it's round it needs to be measured precisely to determine what's a friction fit. Once installed it cannot be allowed to moved as the valve opening is precisely placed. The inside of the bore is determined by using a high speed grinder to take the .008 excess down to the right size for barely a slip fit. That is a slow and tedious procedure. You only have one shot. Anything over and you have a vacuum leak. Here's the rub. Inserting a standard round bronze bushing into the aluminum housing it will oblong the bore, meaning that the CNC'd bore will be an instant leak.
The two shops worked their magic and I had a booster that held 26 inches of vacuum.
32895
32896
I was told that his parents had a brake problem, but he had already fixed it by the time the new T-V arrived, hence it sitting for 50 years. Early on there were few spare part available. I asked why the unit was plated, instead of painted, and he had no good answer in his mind. It wasn't until I received the first day of production Mark II photos that revealed that all of the early cars had plated vacuum canisters. The unit I have has a soft zinc coating that was easily scratched in transit to the assembly plant. The first time they got wet they started to rust. Once discovering this flaw they were all painted black. I have confirmed with a major rebuilder that the vacuum chamber was plated inside and out. More about that, later.
My car was fitted with a sleeve-valve unit when I bought it. I believe that the two types of units work great when new, but the sleeve-valve unit started deteriorating the first time it was used. The only difference between the two units are in the means by which you activate the vacuum switch with foot pressure when you depress the pedal about 1/4" What they didn't know is that each time you press the pedal the soft bronze bushing slotted to allow vacuum into the chamber starts to wear as the bronze bushing is designed to be sacrificial. Each time the shuttle that moves turns on the vacuum that draws it into the vacuum chamber that movement under vacuum pushes the displacement rod into the master cylinder, closing the compensating valve, and pushes brake fluid out to the wheel cylinders. When they discovered that each bit of wear created a bigger and bigger vacuum leak each time you pressed the pedal. The change in effectiveness is so subtle that you simply respond with more and more pressure until you unknowingly have no power brakes at all. I believe this is the cause of the perception that the Mark II has bad brakes.
Instead of relying on a near friction fit the poppet valve unit uses rubber seals that positively traps vacuum in the chamber providing stopping power giving the impression that the brakes are "grabby", by comparison to a worn sleeve valve unit. I built two poppet-valve units, one from Mad and one that was a rebuildable core. I have a third vacuum chamber, but it is too badly pitted to be reused as the 6" diameter seal must have a smooth surface to seal against and slide on lubricated by Neetsfoot Oil, primarily used in high end leather treatments, meant to swell and lube the leather seal surface.
I contacted the remanufacturer to see if he could rebuild the sleeve valve and he said no. He said all of the units done by his favorite machinist didn't hold sufficient vacuum. I really wanted to rebuild mine, but no parts are available. In talking to my local old-school machinist and my CNC shop with a Rainman-level programmer they explained that there were actually two procedures needed. The first is to push the old bushing out of the aluminum housing. The hole needs to be checked for roundness. If it's round it needs to be measured precisely to determine what's a friction fit. Once installed it cannot be allowed to moved as the valve opening is precisely placed. The inside of the bore is determined by using a high speed grinder to take the .008 excess down to the right size for barely a slip fit. That is a slow and tedious procedure. You only have one shot. Anything over and you have a vacuum leak. Here's the rub. Inserting a standard round bronze bushing into the aluminum housing it will oblong the bore, meaning that the CNC'd bore will be an instant leak.
The two shops worked their magic and I had a booster that held 26 inches of vacuum.
32895
32896