Treadle Vac failure

nellis003

nellis003

Senior Member
Hi all,

I have a 1958 Edsel Citation, and when I bought it the car had manual brakes. The size of the car makes it very difficult to stop. I found a Treadle-Vac system and rebuilt it because I wanted the car to remain stock.

Took the car out for a drive today and the brakes worked great for about the first ten miles or so, then slowly the pedal got stiffer and stiffer. Now it's like the power assist is completely gone. The other symptom is that the brake lights (which are triggered by a pressure switch on the MC) remain on all the time, like there's constant light pressure on the brakes.

I did a cursory check of the hoses and can't find a leak. Can anyone tell me what the best way to diagnose the problem would be? I was going to check vacuum with a gauge next, but I'm not sure what the reading should normally be.

Thanks for any help.

Nick
 
You need to check the linkage between the pedal and the master cylinder. My wagon had the exact same symptoms, brakes were great until things got warmed up, brake lights stayed on, pedal was high and hard. Found the push rod would not seat all the way down into the piston in the master cylinder. The inside of the piston had a very slight ridge near the bottom of the hole. With everything bolted up it prevented the piston from returning to it's rest spot. Took the push rod off the pedal and ground down the diameter on the end until it went into the piston all the way down and bottomed out. Been fine ever since.

On a side note, the diameter of the power master cylinder is 1/8" smaller then the manual master cylinder.
 
Wow, I never would have thought of that. Thanks! I'm going to be out by the car again tomorrow, so I'll check that and post progress.

Thanks again,
Nick
 
hi. new guy here.
did the m/c rod mod work for you? i have a 58 fairlane 500 with the same problem. i was reading the factory shop manual and it says that the power m/c bore is 1 18 in (m/c unavailable). the manual m/c is 1 in which i had to use. pedal pressure is ok until the pedal gets hard at which time the bellows seems to hang at 3/4 of the way returned. not sure what to do at this point. after the car sits it appears that the m/c piston is fully returned. thanks for any help you can give as i am not familiar with this system at all. by the way all wheel cyls hoses springs and linings are all new.
jim
 
Jim I went through all that with mine (new everything) to no avail. Did you convert from manual to power? The pedal is different between the two. Yes the power master is larger, re-read my previous post and had it back assward....I have seen the 1 1/8" power masters on ebay, not purchased one as I am still manual. The bore difference should not cause the issue you are having, just make the brakes more "sensitive" and require a slightly longer stroke for engagement. Make sure the return spring on the pedal is still engaged as it should be, mine was not, that took care of the brake light staying on issue. Again check all the push rod lengths, the shouldered bolt should go into the pedal and the push rod with no forcing required, if you have to push on the push rod to get it to go in the hole you have found the problem.
 
bill
thanks for replying! my 58 is a factory power brake car. i bought one of the 1 1/8 m/c s from an outfit in min. but the bolt pattern is different to the firewall. i have seen the rebuild kits on ebay but my original m/c was too far gone to hone
i will check the pushrod adj tomorrow . the vehicle sat for a long time so i am wondering if one of the vac valves inside is sticking
jim
 
What you may have had happen is you didn't depress the input rod before installing the equalizer valve in the bottom of the hydraulic fluid reservoir. Then, the first time you applied brakes, you sheared off the tip of the valve stem.

Thereafter, as the fluid in the system warmed up, it expanded and, with the valve always closed, couldn't return into the reservoir. That resulted in increased pressure in the hydraulics, which applied constant pressure on the wheel cylinders and turned the brake light on, as well as applying the brakes without pedal force.

The valve can be removed and checked without removing the T-V unit from the car. Unfortunately, replacement valves don't seem to be sold separately, only part of a $60 rebuild kit. Remove the reservoir cap by turning out the six retaining screws, then use a 1-1/16" socket to remove the valve and seat. Check the length of the valve stem below the valve and seat against a new one. When replacing it, push the pedal down, and then install the valve and seat fitting. When you release the pedal, you should see the valve stem tip to open the equalizer valve. The valve must be open when the brakes aren't applied, and closed (stem perpendicular to the seat fitting) when applied.

If this doesn't describe your situation, it's also possible that the T-V vacuum piston doesn't return to the fully de-activated position under pressure of the return spring inside the T-V. Check to make sure the equalizer valve is tipping when the pedal is released. If it isn't, and the tip of the stem is *not* sheared off, you may have excessive friction in the pedal and piston system.
 
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