Mechanical Power brakes

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Hi — Been reading an old issue of “The Arrow” on the 1933 models and realized they were the first for the new mechanical servo power brakes (with the brake pedal looking like the accelerator).

Just curious if this system ever could fail? It is possible? Would you have any foot brakes at all? The pedal is so small and “fragile” looking, it’s hard to imagine pressing on it hard enough to stop the car manually. If you even could.

For comparison, I drove a 1928 Model 36 once that had the vacuum power brakes fail. I could stop the car but only with enormous pedal pressure. It was a sobering experience.

–Scott

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Hi Scott,

Having driven my ’33 and ’34 in all conditions I can attest to the fact that these are the best power brakes on an old car. The system can only fail suddenly in the same way any mechanical braking system could fail, that is, if a clevis pin fails or a cotter pin drops out, or a cable fails. All very unlikely.

The system is very different from a vacuum “power assist” system – it is a FULL power system. The force for the brakes that pulls the cables on is derived from a clutch that is driven by the drive shaft. All the pedal does is “modulate” or control how hard the clutch is engaged – no power comes from your foot. The clutch wears very slowly, and will give plenty of warning before it needs to be relined. If for any reason the system should fail, the handbrake still works (unless the cables fail). The whole thing was explained in a brouchure “A New Way to Stop a Motor Car”, which was reproduced in Arrow 66-2. The Stewart-Warner explanatory brochure was published in PAS Service Bulletin 1974-1.

Cheers,

Bob Jacobsen

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Hello Scott,

In addition to Bob’s commnets above you should know that the hand brake operates on all four wheels as very nice feature. Use a straight mineral gear oil in the transmission and power brake unit. Do not repeat not use a hypoid gear oil.

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Paul,

Why not 85/140 differential oil? (Can you tell that’s what I’m running in mine?)

Bill

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Scott,

This brake system provides the best braking available on a car until the advent of disc brakes. When my 1936 was in a shop that has done a lot of P-A mechanical work, the owner commented that the brakes on my Vacuum assisted car were “the best he’d ever driven” on a late Pierce. Yeah, they are good, but they pale in comparison to the mechanical brakes on my 1933 1247. That was a big reason I wanted a 1933 Pierce to begin with.

Scott Stastny showed me that the pedal feel is adjustable to avoid premature lockup. Prior to that adjustment the only negatives for me were that they acted too much like an on/off switch and were a bit inconsistent because the clutch lining needed to be replaced.

The 1933 braking performance is far superior to my lighter and hydraulic braked 1941 Cadillac. Except for tire contact area limitation it’s better than my 1956 Imperial with its sophisicated 4 shoe hydraulic drum arrangement. The 1933 braking system was an expensive luxury improvement. This setup is so good that only one other manufacturer used it: Rolls-Royce(and did so through 1954!) You need to be careful that passengers don’t hit the windshield.

While the vacuum system was used from 1936 on, the change was likely due to 2 reasons: 1)It was far less expensive 2)The slight lag in response when the driveshaft was barely moving is a bit ‘off-putting’ the 1st time you park one of these cars…always allow a few extra inches! As more cars became owner-driven later in the Depression the cheaper vacuum system required no learning curve for the neophyte or less experienced driver.

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Amen on the lag time. I restored a ’34 840 in the late 70’s. The brake system was in excellent condition, but I’ll never forget the first time I was coasting along, very low speed, and went for the brakes. Yes, they engaged, but after a lag that had my heart in my throat. It is a great system, just one that takes a little getting used to. Hydraulic brakes, you put your foot down and feel the back pressure. The SW system, you put your foot on the pedal and start “thinking” stop, your foot eases down, and next thing you know, you stop. Really nice, once you get used to it, which takes very little. David Coco Winchester Va.

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If gear oil with a hypoid additive is used in the transmission and brake unit (’33 to ’35), it will gum up the linings in the brake unit.

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Not only were the power brakes the best America had to offer,the size

of the brake drums and lining was far superior to the rest.Regarding

rear brakes,Super 8 Packards had 14″ drums,30.25 length.V12 Packards had

15″ drums,32.25 length.Duesenberg had 15″ drums,28.75 length.Cadillac

V16 had 15″ drums,29+ length.

Pierce had 16″ drums and 38 lining length.Eights and V12’s both got

the same brakes.In ’33 to ’35 the heaviest production V12’s were about

500 to 700 pounds less than a V16 Cad. or Duesenberg.

Tony Costa

My source on Brakes is Motor’s Handbook pg.122.Weights:P.A.=NADA pg.509.

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