Bakes in my 1935 Dodge truck work weird comparing manual/power

This is an unusual problem on my near complete 1935 Dodge truck. First let me say what I have installed for brakes: a new swing pedal with a 5:1 ratio and a 7" single booster with a corvette 1" bore master cylinder with 3/8" x 24, 2 port outlets. And for the front wheels are original 1935 Dodge truck brakes with new wheel cylinders and new brake shoes. For the rear end is a 70 Nova with self adjusting brakes with new wheel cylinders. All the lines are new 3/16" along with all new brake hoses. This is my problem: if I press on the brake pedal with the engine off (no Vac) the pedal starts to have back pressure fairly soon and does not go all the way down. With the engine running and Vacuum on the booster the pedal goes down fast with no back pressure until it seems to go to the end of the master cylinder, not to the floor. If I drive it with vacuum hose disconnected from booster and plugged, the truck will stop but with a lot of pedal pressure (manual brakes). With the booster hooked back up and driving the truck, when I press on the brake pedal it goes to the same stop as before and not stopping good. I clamped off all the brake rubber lines and the pressing the pedal stop near the top (correct). Then I took one clamp at a time, the rear worked correct, reinstalled clamp. I took off one at a time the two front clamps and they both had the pedal go to same stop as before, (incorrect). Am I missing something because I have pressure reverse bled all the lines, no air. If I can't get to work, I guess I'm going to have to find some disk brake conversion kit $$$. :confused:

Difference in volume of front wheel brake cylinders vs. rear wheel brake cylinders, vs. master cylinder? I'm no hydraulics engineer, but it seems to me that if you get the low pedal with the rear lines clamped and the front lines open, vs. the front lines clamped and the rear lines open (normal pedal), the problem lies mainly in the front brake cylinders, or perhaps the whole front brake assemblies. At least, that is where I would start looking. An adjustable proportioning valve might be needed, perhaps.

,.

Quote:

---

Originally Posted by Rrumbler

Difference in volume of front wheel brake cylinders vs. rear wheel brake cylinders, vs. master cylinder? I'm no hydraulics engineer, but it seems to me that if you get the low pedal with the rear lines clamped and the front lines open, vs. the front lines clamped and the rear lines open (normal pedal), the problem lies mainly in the front brake cylinders, or perhaps the whole front brake assemblies. At least, that is where I would start looking. An adjustable proportioning valve might be needed, perhaps.

---

When you say "Corvette" MC doesn't that equate to a disc brake MC? Sounds like you're running drum/drum, but your MC is disc/disc?

Unless you have a master cylinder designed to pump two different volumes, the volumes must match. If the front brakes demand more volume than the rear brakes (or visa versa), the system will not work correctly. The brake cylinder volumes all must match, ie., the rear cylinders capacity is 50cc, then the front must also be 50cc, if one cylinder requires greater than the other, the large volume cylinder will fill at a slower rate and not equal pressures.

Also, their total volume must be less than what the master cylinder can supply or the master cylinder will run out of fluid before pressure is built. I.E., if the master cylinder's total output volume capacity is 1qt, the total volume capacity of all the cylinders combined must be less than (not equal to or more than) 1qt.

Maybe it's me but my point is: if I press on the brake pedal with the engine off (no Vac) the pedal starts to have back pressure fairly soon and does not go all the way down before it gets hard. With the engine running and Vacuum on the booster the pedal goes down fast with no back pressure until it seems to go to the end of the master cylinder, not to the floor. But it will not go any further and does not have the feeling of brake pressure.
Why would there be a difference in using the booster and not?

I’ll take a shot at a few things for what it’s worth. Some of this stuff is pretty basic and you probably already know about it but just in case.

First is adjustment. If the brakes aren’t adjusted properly you’ll use have a long travel on the pedal in order to supply enough fluid to all the wheel cylinders. Rule of thumb on manual adjusters like you probably have in the front is to adjust them till the wheel locks and then back them off 10 clicks. The “self adjusters” on the rear must still be initially set. For the self adjusters on the rear, tighten till the wheel locks and them back them off 12 clicks (you go 12 on these because of the finer (more numerous) teeth on the star wheel.

Some of the rear brakes I’ve worked on also require that the emergency brakes also be connected and properly adjusted in order to get a proper pedal.

In addition to the brakes themselves being adjusted properly you may also want to check and make sure the rod from the brake pedal to the booster and the rod from the booster to the MC are correct.

The comments about the volume of the master cylinder volume are pretty much right on. You may need to step up to an 1 1/8” – 1 ¼” bore MC.

Rogers comment about the master cylinder being for a Drum/Drum setup verses a Disk/Drum or Disk/Disk also correct. You will need to specify a MC for a drum/drum setup (which usually means something from the 60s). One of the main reasons for this is that the majority of the early Drum/Drum master cylinders included a residual pressure valve (s) which are needed on all drum brake axles and usually not used (or one with a lower pressure is installed) on Disk Brake axles. It can get a bit confusing when you start getting into cars around 1968 and later with drum brakes. In many of those cars the residual pressure valve was moved from the MC to the brake proportioning valve.


When you do get a good pedal, don’t be real surprised is it still doesn’t stop well for a while. I suspect that at least the front brake drums (and possibly the rears are used and have been turned. When this happens and new shoes are installed the curvature of the drum is larger than that of the new shoe, resulting in a small contact patch between the shoe and drum when the brakes are applied. As the shoe wears into the drum the circumferences will eventually match and stopping should at least be passable. Years ago a lot of shops used to “arc” the new shoes to fit the drums…….now days most mechanics just look at you funny if you ask them about arcing the shoes, and wouldn’t recognize the equipment to do it if they tripped over it. Hell now days most of the kids don’t know how to work on a car that doesn’t have 4 wheel disks and ABS. :3dSMILE:


.

Thanks Mike for the insight on newer mechanics, it is true to the core. I asked some about my problem, and they could say is 'You don't have disk all the way around'? I have 3/16" brake lines and 1" bore MC, I will try and find a MC with 1 1/4" bore that will support 1/4" brake lines with dual bowls. I will still use the 3/16" line to the rear since I know they are working correctly. I tried a residual valve in the front line, same results. I did find one MC that has 1 1/4" bore and has one larger bowl that the other, large one for the front with 1/4" lines and smaller one for rear with 3/16" lines, think that will work?

" Years ago a lot of shops used to “arc” the new shoes to fit the drums…….now days most mechanics just look at you funny if you ask them about arcing the shoes, and wouldn’t recognize the equipment to do it if they tripped over it."

When I used to get new linings installed they were "fitted" to the drums by free hand sanding or grinding. I still do that on vintage motorcycles. Time consuming but a perfect fit is possible. A lot of desire for detail has been lost to technology.

Brakes could be weak because of adjustment, arcing of shoes, primary/secondary shoes wrong place/ wrong side of car,

You really should use a single resivor master cylinder

If you are using a dual master , do you have the combo valve that equalizes front/rear?

Drum brakes need 10# residual valves to keep shoes near drums,

have you pumped the brakes(used to be common driving practice

did you bench bleed the master cylinder??????

1/4 inch lines won't help you