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chris_58

1958 Cadillac Bendix Treadle Vac question

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Hi there,

My 1958 Cadillac with a Bendix Treadle Vac brake system has developed a very hard pedal with almost no pedal travel. When applying the brakes it is almost as if there is no vacuum assist and you have to apply a lot of pressure (sometimes scary) to get the brakes to work. My 1958 Buick with a similar brake system has a "normal" pedal movement with a nice power assist. When I try to push the plunger up (at the booster, not the pedal), I can not get more than about 1/2 inch travel. Is this indicating a problem with the master cylinder (swollen seal hanging up at port?)?

Regarding the booster:

I checked the vacuum at the hose that goes from the engine to the booster and I have about 14 to 15 in Hg at idle (without check valve). When measured with the check valve in place it goes up to 20+ in Hg when reved up.

When I checked the vacuum container, I used a hand vacuum pump and pumped it to 10 in Hg vacuum and it stayed there stable without loosing vacuum for several minutes (until I pulled the plug).

However, when I try to apply vacuum to the inlet on the booster, I can not build up a vacuum, not matter how hard I try. When I connected the vacuum hose from the engine to the booster without the check valve, I mean to hear a hissing sound (difficult to determine with the engine running).

So I assume that there is an issue with the Treadle Vac, even though it was sent out for overhaul by the previous owner.

So here are my questions:

  • Can the Treadle Vac be rebuild "at home" (with a repair kit for both the main cylinder and the booster including a new leather piston seal) or is this a job for an "expert" rebuilder?
  • Does anyone have any clues on what actually might be damaged?
  • Will a master cylinder rebuild take care of the short pedal movement? Btw: wheel cylinders work well and brake hoses at front wheels were replaced to exclude collapsed hoses from the equation.

Thank you for any hint,

Chris

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I can tell you what I went through rebuilding my Treadle Vac booster for my 59 Buick. Before I rebuilt it, it leaked and sucked some brake fluid into the intake. I got the kit from Kanter and decided to rebuild it myself (big mistake). I have never rebuilt a brake booster before. I started by taking it apart to its basic components, and cleaning it. I installed the kit and reassembled it without using a manual (I am pretty good at remembering how to put things together). When I tested it, the booster worked great (hardly had to use ANY pedal pressure to get it to engage and went to the floor without much pressure)(I did bleed the brakes and rebuilt them before doing the booster). The problem was that for the life of me I couldn't figure out why so much brake fluid was getting sucked in the intake. I took it apart again to look at the 'veins' in the vacuum mechanism and it looked just like it did when I first took it apart. I ended up trading the entire power brake set up for a manual brake assembly. Had I of known then what I know now I would have sent it out to a professional.

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Chris,

Your symptoms indicate the booster has indeed failed and leaking internally. The hard pedal and short travel indicates the master cylinder is ok right now. Naturally I would reseal it when the booster is repaired.

I rebuilt my 55 but have not rebuilt a 58. ONLY you can decide if you want to tackle that, depends largely on your abilities and determination and confidence level.

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Classic50s,

You have most likely figured out by now that the master cylinder leakage was failure one, the mating area should be vented to prevent booster from pulling the fluid into the booster. You actually had 2 problems.

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Hello John,

Thanks for the reply. I feel pretty confident that I can rebuild the brake booster (I had previously rebuilt several carburetors and had good luck). I have a period repair manual at hand (not just the Cadillac manual but a real brake system manual) that explains step by step how to dis-assemble/assemble the treadle vac unit. Just wasn't sure if any special tools are necessary.

Again, thank you for your reply,

Chris

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Chris, If any special tools are required the manual should indicate where. I usually have to improvise on such projects.

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At the time I didn't consider the master cylinder as being at fault. I do recall scrutinizing the master cylinder and booster seals when it was apart. It comes down to I didn't do my research before rebuilding it. I should have got a manual and read up on it before tackling it.

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