i replaced the single m/c on a 64 buick riviera with drums all the way around with a dual master from a 67 rivera # mc66712 from oreilly's. i relaced the front hoses and had to make new lines from the master to the stock 3 way t fitting. I plugged the stock rear output, used a union to extend the new 3/16" made line from the m/c to the rear of the car. Made a new line from the M/C to the stock input on the t for the front lines, even bent in the factory looking loops, also made with 3/16" tubing. the problem is the front wheels lock up with VERY little pressure. 1/2" of pedal movement equels locking up on the gr*** and it feels like the ball joints/front suspension is getting ripped out from under the car when applying minial pressure on the street at idle speeds.I checked the rod depth to make sure that it wasnt touching, and it wasnt. so that leeds me to my question should i maybe increase the front line size? maybe building to much pressure? just to triple check myself the rear port of the m/c is the front output and the front port is the rear line ? correct??
from what ive gathered on the net and in their books its for a drum drum car. but even if it was for a disk wouldn't a disk m/c build even less pressure? 2lbs for disk and 10 for drums?
The line size won't do diddly for your pressure issues. Also as a point of reference - the 2# and 10# RPV's have nothing to do with working pressure in the system. A disc brake working pressure is in the 800-1000psi range while a drum brake system (dual servo) is roughly half of that. Your brakes are designed to work with the same pressure (obviously) since it was a single bore m/c origionally. If your fronts are grabbing - it suggests you lack pressure in the rear of the system ---> re-bleed.
no prop. valve. from the readings on the net no one seems to be using one.i may be just plan wrong,but i dont think it really needs one if the m/c is for a drum drum app. my 61,62,and 63 cadillacs all have drums with no no prop valve.
ryno; Good answer, I have to agree with HemiRambler; try a re-bleed. New lines have an oil resiidue on them that is not too friendly with brake fluid. Give it a good flush and you should be OK.
do you have a booster??? check the rod between the booster and m/c for improper adjustment.if it is out of adjustment the front brakes will lock down.
yes power booster, i check the rod lenght when i first did the mock up. i put some grease on the end of rod and checked the depth, it looked alright.
true, but if it was hitting it at all it wouldnt it be creating alot of drag, at least thats been my findings in the past?
im going to check over everything, diffently not ruling anything out at this point. i never did remove the drums so i dont see how anything could be inside on the shoes or liners, the brakes worked good when the old m/c was on there.all i replaced was the front hoses and installed a dual m/c with new hard lines.
Well changing hoses and lines should not have anything to do with the problem. So it has to be that $*&)(%() dual master cylinder. Are you getting a good fluid flow from both front and rear bleeders? I would un-bolt the new M/C and pull it away from the booster ,put in a 1/8 inch worth of washers and re-bolt it up. Re-bleed and see what you got. All M/Cs are not the same where the push rod seats. I have seen many times problems from not MEASURING the depth AND diameter of the piston at the push rod seat when making change overs. With a booster you can not see and feel for the bit of clearance needed.
im going out right now to put a caliper to the depth and dia. of the m/c and give a rebleed a try. yes the fluid was really good and im pretty confident theres no air in the system, but as stated earlier who knows now? time to start over and recheck everything.
I had the problem of front brakes being sensitive when I had a disc/drum master on a drum/drum set-up. It went away when I changed to a drum/drum master.
The only reason that could have happened is the disk/drum M/C had a smaller bore than the replacement drum/drum. In any dual M/C the bore is the same size for both ends which means the same output pressure on both side for a given pedal pressure. Remember a smaller bore cylinder moves less fluid but at a higher PSI [line pressure] for a given pedal pressure.
It sounds like a disc/drum master cylinder is causing your problem, double check that the master you just put on is for a drum/drum car. Napa's online catalog shows both disc/drum and drum/drum master cylinders for the '67 riviera, you might have received the wrong one by mistake.
well with my findings today i would say that the original m/c was not working right. as crazy as this may sound, i went out to rip it all apart and recheck everything.i backed it up to pull it into the shop and noticed the brakes felt pretty good. pulled into the shop, check to be sure nothing was leaking. looked good , so i decided to take it out on the street and feel it out again. it started to shake in the front again, so i threw it in reverse and gave it some more speed, with some agression, and whouldn't you know it started to feel atlittle better.back into drive and some gas, it started to shake alittle less. i think the front shoes havent been working for who knows how long. they seemd to seat in and started to work like normal after a few 15-20 mph stops.i plan on rebuilding the whole front end and then will replace everything else in the brake system then. thanks to everyone for ALL your input and advice.
I'm aware that the bore size is the same front to rear. But if there is no difference in the way they operate explain this. Why is the master cyl. chamber for front disc brakes larger than those for drum brakes master? Tell us why if there is no difference in the way they work why the manufactures build masters for drum/drum, disc/drum and disc/disc.
the reservoir is larger for disk because the capacity it takes to operate the puck for disk system versus the small drum cylinder.
Phat rat - the chamber size is different because when the brake pads wear the realtively ginormous disc brake piston moves out - being of such a large diameter a LOT of fluid is needed to be displaced - that fluid comes from the m/c - having the chambers a differnt size is simply done to balance the volume used as pads and shoes wear. They do basically work the same - but there are oc***ionally different components inside - the drum m/c often have the 10psi RPV's built in. Rarely did the factory use 2psi RPV's on disc brakes because the physical location of the m/c was high enough ABOVE the caliper that there was ALREADY some pressure in the line. EDIT: the pressure in the line was there to keep the pads in close proximity to the rotor so that there was a minimum of pedal travel - again because of the relative ginormous diameter of the disc brake piston. In the drum brake setups utilizing the 10psi RPV's - the main reason for that pressure wasn't to eliminate excessive pedal - but to prevent the wheel cylinders from ****ing in air when the pedal was released and the return springs shoved the shoes back into position - in lieu of RPV's the factory also used a "lip spreader" to keep the wheel cylinders lips up against the bore - again to prevent ****ing in air upon brake release. You CAN mix and match drum/drum m/c and disc/disc m/c - but you REALLY need to understand teh inner workings and make sure your system DOES function as origionally intended.
If I understand your comments correctly on the whys, your confirming what I said earlier about the problem of a disc/drum master being used on a drum/drum setup then. More fluid being pushed causes the front brakes to come on much quicker when using the disc/drum master on a drum/drum system. Or am I wrong?
You're basically pushing the same amount of fluid (more or less) - the disparity in m/c chamber size comes into play long after - as pads and shoes WEAR. Lets say your rear shoe wears an 1/8" of an inch.....add that to the wheel cylinder diameter of lets say 7/8" and you get a VOLUME of .07516 cu in. (A= Pi * R^2) Compare that to a disc brake that lets say has the same 1/8" of wear -but has a 3" piston - the volume is .8835 cu in - or basically 10 times the volume. If the chamber was left the same size it would function the same, BUT as the pads/shoes wore the disc reservoir would go empty while the drum reservoir would barely have "lost" anything.
Sorry to raise this thread from the grave. I have too been thinking of a 67 Riv master cylinder for my 63. Only trouble is you can't look at one here because they are all special order. I believe the bendix booster, drum/drum master is 1" bore and has a deep rod hole in the piston like my original master. Is that the one I would need? The rear most port and resevour is for the front, correct? Thank you much for any help
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