Good morning fellas Gonna try and keep this short. 1956 pontiac chieftan-treadlevac brakes ok i bought this car recently. disc brakes already on the front with the original treadle vac ok so they have worked really well. Car has always felt super safe. Then the pedal started to stick down and had to pull it back up with my foot. So i seen on an olds site a guy used a 1" remote master with a 7" booster and it worked well for him. So I ordered the master cylinder. I got a 7" dual brake booster. Installed it and the brakes work now but the pedal is very hard to push to get the car to stop. I was hoping maybe someone else had done this? Im to the point of rebuilding my original master and calling it a day. I really believe the issue is pedal ratio. Most brake setups call for 4:1 for power brakes I think. With the original pontiac its 1:1. I was just looking for some tips on this. Thanks guys
To get a lighter "feel" on the brake pedal, you'll either need to change the pedal-ratio, or change the master cyl bore dia, or change the booster-size(assuming it's actually working correctly now, doesn't sound like it). I don't have my books handy, but there're a few guys on here that could get you really close. Kerry is one, Millian(?) is another. Off the top of my head, pedal ratio of 1:1 doesn't sound right, 4:1 or 5:1 sounds about correct for power brakes, 6:1 or 7:1 sounds about right for non-power brakes. Master cyl bore depends also on what you've got for disc-puck-dia, + rear cyl dia. Hydraulic sizing isn't that hard, but there are compromises to be accepted. Pretty sure anyone helping will need more exacting info on what you have now, to be able to get you to where you want to be. Might want to list the actual pedal-ratio, & there should be a hole or "peg" on the pedal for optional-braking, what calipers you have along w/the bore-size, what rear brakes(guessing drum?) & the cyl bore-size, also which/whose 7" dual-booster & how you have it hooked up. Don't forget to list specs of master cyl whose/which make & bore dia(s)(I'm going to guess 1" but maybe it isn't & is it stepped?). This is going to be needed info. Marcus...
Ok. So on these pontiacs. The pedal ratio is 1:1. 1" master cylinder 1" rear wheel cylinder 7" dual diagram booster I've tried 2 new boosters. Yes they are Chinese. Not sure on the front calipers.
They are new. Not sure how they could affect lack of brake pressure which is what I think is the problem. If the pedal ratio was higher I think it would work. Looks like I'm gonna rebuild the treadle vac.
And just out of curiosity I tore the original treadle vac down. The bore size on the original master cylinder is just under 11/16.
On the 56 Chev Treadle-vac there is a "multiplier" linkage under the dash that connects brake pedal to the Treadle-. I've removed mine, and will be just drilling a new hole on the brake pedal arm, to get the 4:1 for my more modern power booster/cylinder. Not sure how the Pontiac compares?
I wish the linkage was more like that. The pontiac unit mounts down to the "toe board " I think they call it. The pedal actually mounts directly to the push rod on the booster. So you gain no leverage at all
The bigger the wheel cylinders or the smaller the master cylinder bore the easier the push on the pedal will be to stop the car. The treadlevac got by with 1 to 1 pedal ratio because the master bore was 11/16".
That's exactly what I'm thinking. I don't think there is a master cylinder with remote fill like what I have out there.
I have a 1" bore remote fill master now. But I'm not sure there is a smaller remote fill available. I wanted to go to a dual master cylinder for "safety"
The Treadlevac is a system of it's own. To go to a dual master you will need to re engineer a swing pedal with the correct leverage to work with a 1" bore dual master cylinder. Maybe a firewall mounted master like most cars had, or look at what Pontiac had if the car had manual brakes. If you could find the stuff.
The pedal ratio on these systems is radically different than conventional manual or power brakes. Without major modifications, this pedal arrangement simply will not work. Common conventional pedal ratios ar 5:1 for power systems, all the way to 7:1 for certain manual systems.
Yeah Gimpy I'm with ya. The reason I attempted it is because a guy on an olds site did the conversion the same way and was happy with the outcome. Anyway I think I'll order a kit for the original booster Monday.
Other makes with the booster on the firewall, and a top-swing pedal are somewhat easier to convert. It's still not a walk in the park, but there is more room to work with up top. With a bottom-swing setup, you just get closer to the ground. I've seen a few under-floor boosters take a hit on a speed-bump or transition.
I hear ya. See the thing is on these pontiacs it's not under the floor. It's on the toe board angled down directly under the steering column. Now on my 50 pontiac I took off the factory heater/defroster and hung a swinging pedal there. I don't want to do that with this one cause everything works on the car. I just wanted "better" brakes.
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