Hi all, I am having trouble with the brake system I am building for my 39 Plymouth and am looking for some advice. On the front I have the disc kit from Charlie Akers and the rear-end is from a 94 mustang with disc so I have disc brakes all around. I purchased a cheap, 67-76 corvette style, 1 bore, manual disc/disc MC from Advanced Auto Parts (Cardone 101371). 3/16 line all around with one 2lb residual valve on the front circuit and one for the rear. My problem is I have a very spongy pedal and it goes all the way to the floor if I press hard. The rear brakes seem fine because when I plug the front port on the MC the pedal is rock hard. If I plug the rear and just run the front, the pedal goes to the floor. If I plug both ports, the pedal is rock hard and does not sink any. I know the first reaction is air in the system but I can guarantee there cant be any in there. I bench bled the master, bled all 4 corners with a Motive power bleeder and again using the regular method with speed bleeders. I removed all 4 calipers and bled them in different orientations while tapping on them with a mallet and it didnt help at all. I havent driven the car yet but it doesnt seem right for the pedal to feel so weak. I do not have a disc/disc combination valve and am wondering if that is my problem. I didnt think I needed any valves because my master is higher than my calipers (put on the residual valves just to try something) and the calipers on the rear have a much smaller piston than the ones on the front. Even if I do need additional valving, I didnt think the effect of not having it would be a spongy, weak pedal. I just want to check with you guys before I spend 100+ dollars on the combination valve and see if there is something I have overlooked or could test further. I also tested the front calipers by pinching off both rubber brake lines and tested them one at a time. With both pinched the pedal is a rock. If I un-pinch one caliper the pedal goes about half way down. When I un-pinch the second, the pedal goes all the way to the floor. I even took the front caliper off, blocked it with a piece of wood, and connected it via a short line right to the master, plugged the rear and the pedal still went to the floor. It does this will both front calipers. I adjusted the push rod several times to make sure there is some play and the MC piston can return all the way back. The pushrod appears to travel plenty and almost bottoms out in the master when the pedal goes to the floor. Thanks in advance Brett
I think that you need a proportioning valve. This reduces the pressure to the rear brakes. You could plumb in a universal one or try something off a 4-wheel disc brake car, like the same year Vette master that you have. I tend to doubt that you have air in the system, but the master is made to be used with a proportioning valve and without it, it is sort of freaked out. Bob
Then the blame must lie with the "CHEAP", your word, master cylinder. If you are building a car use only NEW brake and steering wear-out components. Why jeopardize your life and worse, the life of anyone else, by being a cheapjohn and trying save a few measly dollars?
Hey there! I just put a four wheel brake system on my 57. Had the same problem. The corvette master cylinder isn't the one you want. I tried one on my car and had the same problem. Master power brakes sent me one that sends more volume and that fixed it. I'm not running a proportioning valve either. 57 4speed
I've seen more than one brand new junk master cylinder. Bench bleeds ok, then goes to crap when trying to bleed the car.
are the front caliper pistond the same diameter as the rear pistons? if not, the volume of the caliper piston bore is larger in the front than the rear.
I have a master cylinder from mp works great with mustang 2 front lincoln versalle disc rear. did you install the front brakes on the correct side? If the bleeder is not in the up position you will never get them to bleed properly. I have seen calipers installed backwards before.
Also do you have a bit of free play in the push rod to the M/C. If not and the piston in the M/C is depressed a bit can give those symtoms. And I've seen more than one new M/C that was defective,especialy rebuild ones.
Are the bleeders on the very top of the front calipers? If not, you could have some air trapped in the calipers still. If the front calipers are installed at a different angle than they were originally meant to be (or worse upside down), then you'll have to rotate them so the bleeder is the high point to be able to get all of the air out. Is it possible you have some hoses up front that were not really designed to be brake hoses and were meant for some other purpose, and they're just swelling up like balloons when you stomp on the pedal?
I tend to agree it is the volume the front calipers are using. Try a MC with more volume for the front. Basicly you are running out of fluid before you fill the calipers.
I don't know if my '38 Chrysler spindles are the same as your '39 Plymouth, but when I put Charlie's disc kit on my '38 Chrysler I had to reverse the calipers ('80's GM type) left-to-right in order to get the bleeders at the top. If you put them on the "Correct" sides, they'll be upside down and trap air at the top.
One other thing to check is your flex lines. I've seen rubber replacement flex lines blow up like balloons when you stand on the pedal, but always from cheap-o brands. I've gotten to where I just don't do the rubber flex hose thing anymore. Braided stainless or nothing at all for me. If anybody bitches about your use of braided lines (but somehow miss the four wheel disks), kick 'em in the nuts and ask them what else they see that pains them while they're down there.
I sure you haven't driven the car yet with "bad " brakes but I had brakes do the same to me and after driving the car around the block and usin the brakes(carefully) They improved alot. Try it. Normal Norman
Thanks for the responses guys, I have really been going crazy trying to figure this out. I'm thinking the $29 reman MC from the local parts store is probably my mistake. It seemed to test OK, when I block both ports the pedal was rock hard and didn't sink any but there must be more to it than that. I'm surprised it doesn't have enough volume though because the thing is huge and when I was originally researching this upgrade I was told by a couple of people, including Charlie and Ply-do, that I could probably get away with using the stock 39 MC... and it is tiny compared to the vette MC. The front caliper pistons are much larger than the ones on the rear. The fonts are from Charlies kit so I am not sure what they specs on them are and the rears are what ever came stock on a 1994 v6 mustang. I'm glad to get some input from someone who is successfully running Charlie's kit and knows the type of calipers I have. Funny you say that because I also had to do that same thing, so technically my front calipers are reversed but that was the only way to get the bleeder on the top. I also removed all 4 calipers and power bled them in different orientations trying to get any trapped air out. Another vote for MP. Thanks 4speed. I will call MP today and order a new MC. I had to custom fab the bracket so hopefully the new MC will mount about the same as the vette style. Thanks for all of your replies, I will follow up on this thread after I get the new MC installed. Hopefully I will be done with these damn brakes and can start moving forward on the rest my project. Brett
I'd spend a lot of time working on bleeding the front calipers before spending any money on parts! You have the symptoms of air in the front brakes.
A friend of mine did a four-wheel-disc conversion on his '39 sedan, he fought the same types of concerns for a year. It finally came down to the TYPE of calipers he installed vs. the TYPE of M/cyl........he had "low drag" calipers" and a standard type of master cylinder. The later model "low drag" calipers have a piston/seal design that will retract the caliper pistons just a little bit as soon as the pressure drops.......just to pull the pads away from the rotors so they don't create "drag" (and ruin your fuel economy). When you re-apply the brakes....it required quite a bit more fluid movement before actually reaching pad-to-rotor contact...thus, he had a low spongy pedal. A switch was made to earlier "non-low-drag" calipers and like magic.....he has awesome brakes. This stuff sometimes is like "black magic" trying to match up all the pieces.
That's interesting because when I was testing the calipers with a piece of wood in them I noticed that the piston on the caliper would pull back slightly when the brake pedal was released. If I pumped the pedal up and down the caliper piston would move and back and forth at the same rate. Do you think it would be better to get a new MC that could accommodate my type of calipers or replace the calipers? I more hesitant to replace the calipers because they work with my current brackets and I don't even know what model car the calipers came from, or what else I could use. I'm looking at MC part #MC390545M on MPs website. Is this the one that will give me more pressure/volume? It is a 1 1/8" bore and I thought 1" would be better but maybe that larger bore is what I need to fully actuate my calipers. My whole car is cobbled together from various parts and an appropriate nickname for it would be black magic. Thanks Brett
The correct m/cyl has a "quick take-up" piston of large diameter that allows the right volume of fluid to move before actually getting some brake action going. Late model Camaros and others have this feature. Maybe that's the master cylinder you need?? This is from MP's website........<TABLE cellSpacing=0 cellPadding=0 width=619 border=0><TBODY><TR><TD width=595>Disc Front & Drum Rear: Manual Brakes with No Pedal </TD><TD width=13></TD></TR></TBODY></TABLE><TABLE cellSpacing=0 cellPadding=0 width=619 border=0><TBODY><TR vAlign=top><TD width=11></TD><TD width=595> Wrong calipers If you are using a low drag or metric GM caliper without the correct quick take up master cylinder you will never get a pedal. In 1982 GM started using the smaller metric caliper because it's design retracted the caliper piston up to .125" ( a regular caliper retracts 2-4 thousandth's of an inch). This is a lot and without the correct quick take up master to move the caliper piston sufficiently your brake pedal will be extremely spongy. The metric low drag caliper can be identified by a chamfered leading edge of the square cut groove for the piston seal. </TD></TR></TBODY></TABLE>
I just wanted to update this thread and let everyone know the progress I have made on my brake problems. Which is none. I ended up purchasing a 1 1/8 bore MC from mpbrakes to replace the 1" bore I was using. It didn't make any difference and the pedal still goes to the floor. I actually drove the car around the block to test the brakes in a real environment and when the pedal goes all the way to the floor, the car slowly rolls to a stop. I then replaced one of the calipers, same problem, and then replaced the other, same problem. I then replaced the original brake light switch junction block with a plain 3/16 tee and that didnt help anything either. I talked to Charlie Ackers and he couldn't explain the problem but told me the calipers are from a 1978 Camero and are not low drag. I know it seems like air in the system but I don't know what else I can do to remove it. I used a Motive power bleeder, removed all 4 calipers and blocked them with wood. Bleed a gallon of brake fluid through them while rotating the calipers and tapping on them. Then did more bleed using the standard pump and hold method and the fluid from all 4 calipers seems perfect with never a bubble in sight. The rear brakes again seem fine because if I put a plug in the front port the pedal is rock hard. If I move the plug to the rear port, the pedal is soft again.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o> <o></o> The only thing I can think of is to buy a disc/disc combination valve. I didn't think one of those would solve the issue of no brakes but I am completely out of ideas. I can post some pictures of my setup if anyone thinks that will help.<o></o> <o> </o> Thanks in advance for any advice.<o></o> Brett
Have you checked your flex lines yet? If not, do it, it may be your problem (especially if they are the same ones that came off the Camaro, IE old as hell). If those are good, check for loops in the brake lines. I had a buddy that had spongey rear brakes, and it turned out to be a combination of a ballooning flex line to the rear end, and two bigass loops in the hard line right before it went into the flex. We replaced the flex line and the problem got better, but didn't go away. After we removed the loops and bled the system again, it was hard as a rock. Check your hard lines for loops or crazy high spots that could trap an air bubble. A loop or a sharp high spot will let the air stay in the line, even when you bleed it (the fluid just skirts past the bubble when you bleed, but it'll compress when the lines are pressurized, giving you a soft pedal feel. And lastly,( it seems like a long shot) but did you maybe mis-match the width of the rotor to the caliper? If you've got a caliper that fits a 1.25" rotor, and you have it mounted on a 0.85" rotor, no master cylinder in the world will pump enough fluid to take up all the slack in that system and still provide line pressure to stop. Try pumping the pedal several times and then seeing how it feels. If it pumps up and gets solid after a few strokes, you probably have a clearance issue between the pads and the rotor ( could be because the pads are wrong or worn out, or because you've got the wrong caliper for the rotor, or because you need a residual pressure valve, or any of a number of other things). At least at that point you'll know it's probably not just air trapped someplace.
I'm in a similar situation here,brakewise and I've learned a lot from this thread! Thanks for all the info!
I just wanted to update this thread and let everyone know I finally found out what the problem was. As it turns out, I was not getting enough push rod travel. Even though when I measured the travel and compared it to my dads disc brake setup on his 57 Chevy, it looked like I had enough, but didn't. In the end, I needed about 1/4" more travel than what I was getting. The last 1/4 inch of rod travel is where the brake pedal really gets firm and now I can lock the brakes up with ease and the car stops on a dime. I had to adjust my brake pedal all the way up. I gained about 2 of pedal travel, which equates to about 1/4" of push rod travel. I'm using the original pedal assembly and it is kind of at a funny angle now but at least I've got brakes. My setup is odd now but it works so well I don't think I am going to change it. After I ordered the $120 MC from mpbrakes and it didn't solve my problem I went to the junkyard and pulled a power MC off a 1994 mustang with the combination valve, $12 (I have a great u-pull junkyard 5 minutes from my house). After I plumbed it and still had problems, I turned my attention to pedal ratio and travel and uncovered the real issue. I've read everywhere you are not suppose to use a power MC with manual brakes but it seems to work fine for me. The other reason I think it will be fine is my dad has built three Factory Five Cobras over the past few years and with those kits you use the MC from the donor mustang without the booster. The mustang MC is a Bendix unit with a 1-1/16 bore. Don't know how shallow the bore is but my push rod will certainly not fall out so I am not worried about that. Attached are some pics of my setup, forgive my peeling paint as I have spilled and squirted gallons of brake fluid around this area. Thanks for everyones help and suggestions. Brett
I did a Model A Vic about 30 years ago with early Chevelle front discs and C2 rear discs. Corvette 1" bore dual master under the floor. No residual valves. Used an adjustable proportioning valve in the rear feed and could never get enough bias change to tell. Brakes were fine. My current '26 track roadster has GM metric calipers front and rear, Corvette 1" bore master mounted in the cowl behind the dash. No proportioning valve or residual valves. A couple thousand miles so far with good responsive brakes. I don't think all metric calipers are low drag but I don't remember which years are not. Charlie
Hot excrement! The old rod (brake pedal that is) is too short. Usually I go the other way and they lock up tight as a crab's ass when they got hot. Bob
You have too much pedal ratio. Figure out what your ratio is, and lessen it by .5 of a point and your pedal will get better. You might have to drill a few holes until it is perfect. Simply weld up the un-needed holes in your pedal. It is not prop valves, M/C's, lines etc. I have run into that problem dozens of times, and fixed it by changing the ratio.
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