My55buick

Need Help with 55 Buick Treadle Vac

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HI everyone, Im new to the forum. I have a 55 Buick Special that I am trying to get back on the road after 45 years. Ive already done the basics, getting the gas tank boiled, rebuilding the carb, rebuilding the fuel pump, got the radiator and heater cores boiled, belts, hoses, and generator rebuild (why the car was parked in 1973). Now I am working on getting the brakes fixed so I can take it for a ride. I grew up working on 60-80s cars and even went to tech school to be a mechanic, so I have a pretty decent knowledge of what to do but this car is really throwing me for a loop. Here is everything I have done as of now. I flushed all the brake lines with fresh fluid before installing new brake hoses, wheel cylinders, and shoes. I sent out the treadle vac to Apple Hydraulics who have rebuilt a few master cylinders for me in the past. They said they have done many Treadle Vacs and it shouldn't be a problem, so I sent it in. They sleeved the master, replaced the piston and shaft, and rebuilt the booster portion of the Treadlevac.  I got it back, installed it in the car, and bled the system. I ran 3 bottles of Dot 3 through the system and have no air I can see. I adjusted the shoes so when you spin the wheels you feel just enough rub that the wheel spins once or twice and stops. My brake pedal still goes almost to the limit and barely stops the car. Being frustrated I decided to look in my Buick Service manual and I found out that I am supposed to set the brake pedal with 3/4 of an inch of pre-load on the master, I didnt know that I am used to newer cars where you want no pre-load. I went back under and adjusted the brake pedal, then bled the system again and now the brakes grab at a slightly higher pedal height but the brake pedal is still almost on the floor to get any response. What am I missing? I wanted to try posting here because every other forum I have tried just keeps trying to talk me into swapping to a manual master cylinder under the floor or power brakes on the fire wall and I am really trying to preserve originality.

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Apple has a good reputation but a treadle vac has a displacement type master cylinder where the piston does not contact the cylinder walls. It does not require sleeving and, since it is tapered, is not sleevable. On the outside chance that apple removed the taper in the bore and straight sleeved it that quite possibly altered the piston's fluid displacement schedule accounting for the low/weak pedal.........Just saying...........Bob

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Wow Bob I hope I didn't pay $750 to have my master cylinder destroyed. How difficult is it to take just the hydraulic portion of the treadle vac off to check?

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It's unlikely they did that so before I disassembled the unit and likely voided whatever guarantee there is I think I'd give them a call and discuss the "sleeving " of the cylinder. You mention that you have a shop manual. Review the power brake section to give yourself the knowledge to ask intelligent questions..........Bob

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Did you first bleed the master cylinder? Another (maybe stupid) question: is your car equipped with a Treadle Vac booster or a Hydrovac system which is entirely different? If I'm asking that is because 1954, '55 and '57 Cadillacs had the Hydrovac system; however they had the Treadle Vac system for 1956 & '58. As your car is a '55 model, Buick had maybe the same system as Cadillac. This would also explain why the master cylinder was sleeved which is not possible from design with the Treadle Vac master cylinder.

Unfortunately, I don't have Buick shop manuals to check.

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My car is a treadle vac set up. I just can't remember if it's the Moraine or Bendix version.

 

I didn't bench bleed only because I couldn't figure out a safe way to hold it in a vice. So I pedal bled the system without going all the way to the floor and once all the wheels had steady fluid, I did the trick where you loosen the brake line at the master and then push the pedal down and then while it's down tighten it back up. I did it repeatedly but go no bubbles from the master cylinder. So I assume there is no air. I was thinking of trying to make a fitting for the reservoir and use my pressure bleeder to try and see if there is any air that's bound somewhere, but I really think the system has been thoroughly bled.

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My 1957 Pontiac has a Treadle Vac and despite my long experience like yourself it caused me trouble after doing my own rebuild.  I sent it to Midwest Power Products at www.midwestbooster.com and it has been working great for several years.  I was very impressed with the owner’s knowledge so if you need a different expert you might check with Rich at 877-966-0550, good luck, Todd C

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Would you be after knowing this trick?

 

Clamp all hoses just enough to stop flow,

then slack one clamp off just enough

to act as a one way valve

while you bleed it's downstream nipple?

 

Repeat for other corners.

 

Please remember to take all clamps off afterwards ?

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I've never tried the clamp trick. Are you saying I should try clamping the rubber hoses and then bleed just 1 wheel at a time?

 

 

The brake pedal is soft with the car off...

 

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