I figure I'll ask. I got this 52 Studebaker running, everything in the brake system is new and engineered to the best of my ability. Still, it takes a whole lot of leg to whoa the car, and I absolutely cannot get any wheel to lock up. Maybe I'm spoiled by my newer, power braked daily driver. But I think something is still not right. 1969 ford 9 inch at the rear, stock drums. Adapted 78 Impala Chevy discs on the Stude spindles up front with a common kit. There is a residual valve in the line to the rear, 2 or 10 lb, can't remember but it's what you are supposed to use. Homemade pedal is 6:1 ratio. Pushrod is adjsutable. Master Cylinder I picked was 78 Chevy truck that had factory non-power disc/drum arrangement. 1" bore, napa 10-1581. Firewall mounted. The car does stop. But not in any hurry. Total pedal travel is maybe 4 inches, and here's a test I made. With the wheels off the ground and a helper spinning them, I start to depress the pedal. About 1/2" down the fronts begin to drag and stop. I can get much farther, about 70% of pedal depressed, before the rears start to drag and stop. Front brakes acting first seems normal, but why so long before the rears act? The pedal does not ever get to the floor, so I don't think adjusting the pushrod will do anything but raise or lower where the pedal starts. I should add that there is a combo valve in the system, and i lied, that piece is not new. Came off a 78 Chevy truck that had good brakes, although they were power. Do these stock combo valves vary inside? maybe buy an adjsutable one?
The car does stop. But not in any hurry. Total pedal travel is maybe 4 inches, and here's a test I made. With the wheels off the ground and a helper spinning them, I start to depress the pedal. About 1/2" down the fronts begin to drag and stop. I can get much farther, about 70% of pedal depressed, before the rears start to drag and stop. this sounds like a proportion valve issue..or lack of
Could be a dumb question, but are you sure the safety valve in the combo is centered? If it shuttled because of a leak or something (air in the system) it will close off one set of brakes. Also, are the shoes adjusted snug against the rear drums? Slack there will cause a major delay in braking, even if the fluid is getting there. From my experience setting up brakes in race cars, that 1" master is on the large side for a 6:1 pedal, but if you've got a good leg, you should still get it to lock at some point. If the master wasn't originally on the firewall, are you sure the firewall is braced well enough so it isn't flexing and using up your pedal effort?
I always use a 7:1 ratio on non-assisted brakes, also I always make sure my master bottom's out , before the pedal hit's the floor. What size line is going back to your rear brakes from your combination valve and why the residual valve with the other combination valve?
I should add that there is a combo valve in the system, and i lied, that piece is not new. Came off a 78 Chevy truck that had good brakes, although they were power. Do these stock combo valves vary inside? maybe buy an adjsutable one?[/QUOTE] Get rid of that 30+ year old combo valve. Yes, they vary in operation depending on the vehicle they were intended for, and it wasn't a '52 Studebaker! Add the fact it is OLD and not repairable. Just install an adjustable prop valve, keep the 10 lb residual to the rears, and add a 2 lb residual to the front discs only IF the master is floor mounted below the calipers. By the way, the differential pressure switch, the thingy with the wire connector and part of the combo valve, operates a dash light when shuttled, but cannot/does not shut off line pressure. It simply lets you know if a hydraulic failure or problem is happening.
7:1 is a pretty commonly accepted ratio for the brake peddle. There are other issues beyond that. Look for a kinked line, or improper line size. As has been mentioned a proportioning valve is probably on order and I might check that combination valve for proper function. Are you using a NEW master cylinder or a new rebuilt master cylinder? I had a Buick one time that we literally replaced everything except the master cylinder. When I got the car the master sylinder had just been replaced with a rebuilt one. The brakes stayed sluggish functional but sluggish. Finally pulled the master cylinder and tore it down, it had a little crack in the cylinder bore.
Guys, thanks for the quick answers. The rear shoes are adjusted, I think I'm fine there. System is bled, of course. Firewall is seriously braced, no movement going on here. I cannot give brake line size by memory, will have to check tonight. The holes out from the master cylinder are 2 different threads, so I used the line that went with the fittings. I should check. School me on the combo valve. I didn't hook the pressure switch up. Any way to check if the valve is OK? What is the rubber covered "button" on the end of these things, and what is it for?
Special master cylinder nuts are available from most auto stores that have 3/16" IDs with various ODs and threads, just like all factories use. Combo valves come in three basic designs: The first is a one-way (not really a combo, used mainly on drum/drum systems) and is just the pressure differential switch, with a wire going to a dash-mounted warning light from the switch, which shuttles when a 400 PSI or so system pressure differential occurs. This switch DOES NOT BLOCK FLUID FLOW by design. It CAN rarely leak internally, which would allow the primary and secondary systems to merge. The second style is a two-way, and has a fixed proportioning valve included, plumbed to the rears. The third style, the three-way, also has a metering or hold-off valve that goes to the front discs. This valve "holds off" pressure to the fronts until the master cylinder develops about 100 PSI. The idea is to allow the rear drum shoes to overcome their strong return springs and start actual braking at the same time as the fronts. These normally have a little button that by-passes the approx. 100 PSI hold off to the fronts for low pressure bleeding. Im not trying to be a smart a__, but everybody REALLY should know what the heck you are plumbing into your brake systems and how they function. ALL 2-way/3-way combo valves should be avoided on ALL non-stock brake systems, especially 30-40+ year old valves! Factory combos were designed and tested for a certain car/light truck, NOT just any vehicle. The use of an adjustable prop valve and necessary residuals are all you NEED, period! Always try to buy NEW USA-MADE master cylinders. They are not that much more expensive than rebuilt ones. Don't try to save money here!!! But in any case, it is a good idea to disassemble and check ALL master and wheel cylinders, new or rebuilt. I always find new ones clean and ready to use, but most rebuilds are dirty and/or damaged. IMO.
Pedal ratio is meaningless without knowing all the other components. The end result you need is to achieve 1100-1200 psi of system pressure. You can't know this without a guage. You can install one on a tee fitting until you sort it out or you can do the math on all the components. Figure you are putting 125 lbs into the pedal. (use a bathroom scale betwwen you and the pedal or something like it to see what 125 lbs feels like) Multiply 125 lbs by the pedal ratio, so 125 X 6 = 750 lbs. to the piston. A 1" piston has a surface area of .785". That goes into 1 square inch 1.27 times. 1.27 X 750 = 955 psi. You are way short on pressure. To get to 1200 psi, you will need a smaller bore master, say .875". That surface area would be 1 divided by .875 = 1.66, so you can get 750 lbs x 1.66 = 1248 lbs. Go get a 7/8" master. I use the one from a '88 Dodge Daytona.
Low pressure and wrong metering pretty much back up the feel at the pedal, which is horrible. This is a big help. I did some research when I built the system. What I have is based on what I learned at the time combined with no budget to work with. The plan is to have a driver this spring, so I have time to fix it. I will have wasted more money then if I had done it right, and way more time and effort. Dam - been there before! All good, lessons learned here. Thanks Hambers.
This is what napa shows for 88 daytona. It does not look traditional. HA! Little joke, this is about being safe and building good cars. Here's why I do like this a lot and will use it. First, the mounting looks very much like the 70's steel pot Chevy unit I currently have. Minimum of firewall rework I hope. 21mm cylinder equals .826" which should ramp up my line presure fine. And those plastic caps should not leak like the steel lid and bail wire tend to do. I cannot tell by picture which line on this MC is front brakes and which is rear brakes. Can you tell me? I have a master power catalog, who else sells a good proportioning valve?
Summit Racing, Jegs, I suppose that Speedy Bill or any number of hot rod/racing supply houses. I don't have a problem with your master cylinder but I suppose that someone would. If you look around a little bit you can find any number of all steel master cylinders around. That is if aesthetics is an issue with you. Just for information and not to change anyone's direction in their build but one of the things that we did when I was a kid was to take two jelly jars mounted side by side and run them on a balance bar. I suppose that someone thought that up befor us.
The problems with small-bore masters are fluid volume and piston stroke (pedal travel). The master should be chosen from a vehicle close in weight and brake cylinder displacement (size). And the pedal HAS to be able to bottom out (FULL STROKE) the master BEFORE it hits the floor, or the dual chamber design is of no advantage over the old single system. IF the pedal cannot fully stroke the master, either a larger bore cylinder must be used, or the pedal ratio has to be changed. When the bore gets too large resulting in very high pedal efforts, additional ratio is needed, or power assist.
V8 Bob, fair enough. The Stude should be sitting right about 3000 lbs with the motor swap. 88 Daytona lists as 2830. Cylinder displacement, that's a harder thing to determine. I don't see the floor getting in the way, I can adjsut as needed. Here's the car, a few projects left to do. I don't give a turkey what the MC looks like, I want a well engineered car. A plastic resivoir would not be the only HAMB unfriendly piece on it. I'll keep shopping for MC options, but that daytona looks good so far. Opinions gladly accepted.
Hmm. 88 Daytona may not move much fluid based on the small 3/8" outlets. 88 Dakota is a very similar looking unit, with 24mm bore, but has 1/2 and 9/16 outlets like I currently have. 24mm is dam clsoe to 1", but given formula above figures maybe 1070 psi.
The outlet port line thread size and line size it's self have not a F^^^&King thing to do with volume and pressure. The line is already full ya push in 5CC in one end ya get the same out the other.
Couple thoughts; Rears shouldn't take that long to catch up, are the brackets putting the calipers in the right location? If they're offset you could be pushing the piston side pad into the rotor and trying to deflect the the bracket and caliper without putting a good squeeze on the rotor. This would use up pedal travel and pressure without doing much braking. Calipers mounted cockeyed or hung on the brackets could do the same thing. If you're using GM calipers, the master size should be close enough to get them to lock, only another 18# over 125# should get you 1100 psi. Maybe not enough to be an easy driver, but enough to work. Another thing to look at is the pads. If they put the kit together with the absolute cheapest junk they could find, decent pads would make a huge difference.
The 88 daytona unit has almost 1.25" stroke. It moves more fluid at full stroke than the wilwood version and generic copies. It also cost about 1/3 as much and is in stock at every parts house in town. I don't recall which port is which but, you can look up any master you want on RockAuto.com and see what the fittings are, bore, stroke, etc.
The Jalopy Journal
