For the last two days I have been changing the braking system on my 63 Studebaker wagon. I ran in to a big problem.First I`ll tell you what system I have.The system is made up of all new parts and brake lines.Nothing used other then the drums. It has a 8 inch ford out back with new shoes wheel cylinders and the drums are cut. up front I have Jeep rotors with S10 Calipers.All new flex lines.My master Cylinder is a 1975 Ford truck that was for Standard brakes, disk front, drums rear,also new.The brake lines are set up the way the Studebaker came. The front has a T fitting after the M/C that has one line to right side and one to left side.The rear goes from the M/C right out to the rear flex line. Now here is my problem. I bleed the system and get all the air out and the peddle feels good but after you take you foot off the peddle,a short time after the peddle falls to the floor when applied.pump it twice and its back up. hold you foot on the peddle and it stays up.There is no leaks in the system and the M/C has been bench blead twice. Tried changing the lines on the M/C front to back but there was no differents.Can`t seem to find any air in the system. Bleeders on the front calipers are mounted at there hightest point so there no air there. we have check the system a number of times and still you get a peddle and let the peddle go and agian a short time later the peddle falls to the floor on the first pump and after two pumps it fine. Any ideas???
how long has the master cyl. sat? theres been a few bad batches of master cylinder floating around these parts, internal leakage..
Where is the MC mounted? Mounting the master cylinder below the floor positions or below the calipers. Gravity will cause the fluid to flow away from the calipers. by adding a residual valve in both lines it will retain pressure within the lines. I had to pump my brakes at each stop in my 40 pickup because a previous builder failed to install them. Add the valves and the problem was fixed.
you dont mention a proportioning valve . i have never used residual valves either with firewall mount or below floor mount no problems. size of bore has a lot to do with brake performance but dont think it would cause this. sounds like internal byp***ing in the mc
try pumping up the brakes, now hold the pedal with only slight pressure, barely pushing but not leaving off, ever so gently, keep this up for a min or so, if over time the pedal slowly goes to the floor the master is shot.
Okay: As an ex car mechanic, I have pulled as many as 10 newly rebuilt master cylinders off the shelf before I got a good one..The number one way to tell if its worth bolting on the car is to bench bleed it.. Follow the directions to bench bleed it in a vise. If after bench bleeding for a hundred cycles or so you see tiny little bubbles coming up from the bottom ports in the resivour then it is definetly a bad master.. There are other things that can be bad but that is the number one.. It is also possible that you have a very small leak at a fitting that is letting air into the system.
I am thinking that the M/C is my problem too but I had to get some fresh minds in on the problem to see if I over looked anything. It is hard to blame new parts but for some reason I hear more and more about new parts that are junk. I built another system on another car using the same M/C with no problems and the system was built much the same way so I`m thinking I just picked up a bad M/C. But if there is any other ideas please let me know.
If you mean that the peddle rod lets go in the back of the M/C.yes.Not alot, maybe 16 of a inch.Should there be some pressure on the back of the M/C at rest? I`m thinking there shouldn`t be.
Something I've learned on OT cars....... Jack one end of the car up real high. Start with the back. Bleed that end. Let car down & raise other end for bleeding. Sometimes the line geometry needs a little ***istance moving air bubbles, especially the rears. good luck
Those calipers MIGHT be the "low drag" style and your Ford master isn't the right style to operate them. Read this... http://www.mpbrakes.com/uploads/do***ents/lowdrag.pdf
When the thing bleeds down try hitting the brakes and have a helper (your helper, a neighbor of your mother-in-law, if your on friendly terms) try to turn the rear drum, if the drum rotates I would suspect some relationship problem between the rear wheel cylinders and the shoes.
I would say if you are very positive all the air is out of the M.C & lines the you have a bad Master cylinder . You should also have a proportioning valve in your set up too . It has to be a bad M/C or air in lines . I was also a mechanic for many years and that would be my choices for the problem . Even if the M/C is new doesn't mean it's good ! RetroJim
here is one way to check m/c on the car. bleed m/c weather bench or mounted then with slight pressure disconect the line front and rear then plug both ports and then apply foot pressure holding peddle down. if peddle goes to floor m/c is bad.
You need about 3/8" of freeplay at the pedal so the port that supplies 'make-up' fluid will unmask and allow additional fluid in. Just cuz the M/C reservoir is full doesn't mean the cylinder has enough fluid in it. Measure freeplay at the pedal.
After doing some homework and it seems that Bill is on to something. The calipers I am running are the same stile as is this link, low drag calipers.I have looked into it and found that there is a standard M/C for such calipers.A 84 S10 came with both the same calipers as I have and comes with a standard M/C like I want.I ordered it today and will let you know next week how I made out. Won`t have it till Wed of next week.
Hey Fitz.... Yeah, I checked today too. The Low Drag master has a large piston on one side to quickly take up the clearance. NAPA has them but I bet you got it cheaper!!! Heres the bad news for me. I THINK I have the same ones for my off topic project! I'm gonna try them as some people with my style rear disc conversion don't need an adjustable proportioning valve...others need it badly! Thats weird...and it might have something to do with the calipers so I'll try them out but I expect I'll be buying more with my luck.
Well Bill you were right. Went to the local parts store and match up a M/C that the front calibers would work with.84 S10 with Standard brakes. Bolted right up but had to "ajust" the fire wall lip. The rod needed to be reworked but my problem is solved.Car has a great peddle on it now. Thanks for the info.
Excellent! I knew you and Monty weren't messing up the bleeding. You guys have been at it way too long...
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