Coming up on almost 2 years of driving with Ol Bessy (1937 Dodge D5 4-door Trunk Back Sedan)....had a couple panic stops here recently and it highlighted a nagging issue in the back of my mind. Id previously switched to a 1" bore master (From 1 1/8' as the system lacked pressure) with my GM power disc/drum combo but no matter the adjustment they just seemed inadaquate. Not much feel at the top of the pedal, long stroke to get to function (master pin is 20 thou off the piston, etc, etc.) and those panic stops were long ones with me and 4 other legs pushing the pedal without any tire bark. Did a bunch of reading and rearranged a few things. Naturally, checked all the shoes and pads and looked for leaks, etc,....all was well. Went back to my youthful methods of adjusting rear brakes, tossing aside the measuring caliper and after fitting shoes to the drums, adjust them to lock and backed off till just free......that helped but still needed 5 legs for a tire quiet panic stop and still not much feel or action at the top half of the stroke. Went through all the checks on the 7" dual diaphragm booster (thats all that'll fit in the space available), run the pressure calculations (should easily make the 900 to 1400 lbs needed for power discs), and decided to change the ratio from power brake 5 to 1 to manual brake 7 to 1. Same, it helped, but still not a factory feel to the brakes and still needed a lot of effort for what was still a totally quiet and rather longish panic stop from 45 mph. Did a bunch more reading and came across several articles on effeciency of levers. Seems the lever ideally needs to be at 90 degrees to the master cylinder push rod at the mid point of the stroke. Mine was set up with the top of the pedal rearward/away from the master cylinder and the angle significantly reduced on application of the brake. So, I moved the pedal pivot toward the master cylinder and changed up the push rod length so I'm a bit over 90 to start and push thru 90 as I'm applying pressure. Happy Joy! I can feel the brakes engaging nicely in the top third of the stroke and panic stops are one legged and a darn sight shorter than before. Even got some tire noise jumping it full panic.....still need to adjust up the right rear a bit tighter but so much more feel and brakeing power. (Some might argue that locking the rear tires increases breaking distance but yowsa, Ol Bessy stops a darn sight faster and shorter today than yesterday, so......) The original mounting point for the brake lever was at the middle bolt, there was a steep angle to the much longer push rod that bacame shallower as I pressed the brake.....e.g. the brake lever was moving into a position parallel to the master push rod. Not efficient per what I was reading. Now, the brake lever is moved toward the firewall and the angle is a bit over 90 degrees to start with and moves through 90 as I operate the brake. Best overall improvement so far to brakeing feel and performance. (Ratios on the three holes top to bottom are 7 to 1, 5.7 to 1 and 4.6 to 1 but the bottom holes require an offset push rod to keep the master pushrod straight on in the bore.) A side benefit, I can no longer see the brake switch from the seated position......glad I left the wires a bit long.....maybe I can wrap them now! Now only a couple more things to do. 1. Get some steel (and a buddy that can weld for me) and fab up a brake lever that hangs like this and is a bit straighter so it fits my foot and seating position within the constricted space of the pushed back fire wall. This prefab one is okay but still a bit far from the firewall. 2. Start making up door cards and arm rests so I can get the upholsterer working on it. (I still have a lot to learn on making up upholstery panels!)
Locking the rear tires will make the front and rear of the car want to switch places. I speak from experience on that one, happened while in rush hour traffic in my old '60 F100.
I had that issue with a power drum drum conversion on a 65 C10, no proportioning valve and had to loosen up the rears a bit. So far no issues on the Dodge with a gm block in the lines to meadiate timing between front and rear but I'll keep an eye out for her butt to swing around.
I had similar problems with my Torino front disk/finned Buick aluminum rear drum brakes on my Willys for years. I have a '68 GTX dual 1" master cylinder under the floor boards. I adapted a '36 Pontiac forged steel pedal so had good mechanical advantage. My problem was weak early braking. I had to double or triple pump to get good brakes which was exciting @ panic stops! I bled and bled the brakes but nothing helped. I finall had a thought. I had always set up the push rod to be a fraction short of contacting the piston at rest. I changed that adjustment to be about two screw threads into the piston, slightly compressing it at rest. VOILA! got fantastic brakes.
No. It put the components in proper orientation. Probably poorly machined Chinese replacement kits? Adjusting the preset put the lip of the rubber cup in proper orientation to the vent hole and the cylinder started working as designed. If it were going to lock up, would happen on first stroke I reckon. Has been working great for two years since I made the adjustment.
I had the same issue it took too much pedal pressure to lock up the brakes in a panic situation I had fabricated my own ( brake pedal) and it turned out I had the wrong ratio & pivot point Math & understanding of ratios was never a strong point...Gene in Mn
@Sharps40 You need to get somebody to stand on the brake pedal while you look underneath. At maximum braking force the pedal pivots need to be at 90° to the push rod . [And the rod needs to be straight inline to the M/C stroke] This also is the principles behind "Raising rate suspension" on GP cars and also Laid over shocks on a motocross bike. The angle affects the "motion ratio" True..... but this can also be a double edged sword here. Front wheel lock-up can cause you to "Liberace" a car in front of you. There is a lot of math involved [and variables like weight bias, CGH, wheelbase etc] But for an average "DIYer" I suggest the "try it and see" approach. The front and rear bias [wheel cylinder sizes] need to be figured out on a wet road with No proportioning valve. This is because brake pressures and weight transfer are low. [limited by traction] edit: This ^^^^ is where you want the front to be teetering on starting to lock up Once this ^^^ is sorted , you don't alter the bias again, But you add an adjustable "proportioning valve" into the rear circuit to limit the pressures to the rear. Then go out and test it again in the dry [and adjust the proportioning valve so the rear doesn't lock up.] 95% of the time the "proportioning valve" does not come into play because of smooth driving/ braking..........[and limited traction] But one day [while TXTing and driving] you might have to slam hard on the brakes and the rears will try to lock up, but cant.[because of the proportioning valve] If all this ^^^ is too much........ I suggest cannibalizing/replicating the whole braking system out of car with a proven combo.[eg: G body GM]
Rear wheel cylinders with 1/8" smaller bore size will probably be enough to reduce or prevent the rear wheels locking. Always nice to be able to hit the brakes and stay straight.
I suggest cannibalizing/replicating the whole braking system out of car with a proven combo.[eg: G body GM] Yep...close to what I did. Looked at the gm metric caliper years and basically copied the simple non abs systems. Gm metric single piston calipers up front. Dual bowl power disc Mc. 71 chevell 12 bolt w factory drum brakes out back. Gm block/switch in between to mediate front to rear and warn of impending doom.
Vacuum power assist should have been fine at 5.7 to 1 ratio and 1” MC. A 7 to 1 with power should throw you thru the windshield. 7-1 with a 7/8” bore MC is a standard for no power assist.
So....last two days of rain spent continuing the brake work and gas pedals.. The firewall was moved back 4" to make room for the small block.....really compromised foot position, comfort, etc....combined with an enlarged trans tunnel hump, not much foot room left and everything really close to the seat. Kinda knees up when driving. I finally vetted the gas pedal and have a set up that works. Started with a Jeep Cherokee assembly, it was ok, but high enough that my foot bent up toward my shin below 70 mph. Pulled that out and put in a hot rod spoon assembly, height and angle was perfect but the spoon sorta bores a hole in the ball of my foot after a couple hours. So, I took both apart and mated the tradition jeep pedal to the hot rod assembly and chucked the spoon down in the woods. So, that's resolved and I have a comfortable resting position for my right foot while driving. Just left to pull it out for paint, after the brakes are done. The brake pedal that came with the booster/bracket package had a huge rise through a 20 degree up angle, kinda boomerang shaped pedal arm. Having fixed the gas pedal, the back of the brake pedal was now 4 inches above the gas.....quite a lift of the leg to get her stopped. (Boomerang brake would have been perfect though, before the firewall was pushed back.) Anyway, stopped off at Tractor Supply Store and grabbed some 3/8" steel and carved out a straighter pedal arm with a 1" rise through 5 degrees. I set it up for 6.3 ratio. Finally! Room for my foot at both pedals. The brake pedal now sits about 1 inch above the gas pedal and adjusting my sloppy free travel from over 1/2 inch down to the called for 1/4 inch, I'm happy. The back of the pedal arm is about 2" off the firewall at full stroke so no worries bout not having enough room to mash it down. While i was at it, I checked my booster pin to master piston relationship. I wasn't sure how much gap was in there but I felt it was too much and I recalled I had a 40 thread per inch pin.... (.025 Inches linear movement per turn) Without the requsite measuring tools and not willing to wait 8 days for 2 day Amazon delivery, I extended the pin 2 full turns toward the piston, put prussian blue (blue grease) on the pin, bolted on and removed the master andd I had full contact between pin and piston. While the vents looked fine, a drive and some hard stops from 25 mph uncovered much reduced brakeing capacity (more than a change from 7 to 1 to 6.3 to 1 ratio should be), really, it felt like front disc only. So, back in the garage, rear drums were checked for heat, 145F on the passenger side, 110F on the drivers side after the 20 mile jog. Not bad, not dragging but not working good either.....figured it had to be the pin since thats the big monkey business i opted into for the day. Put the camera over the resevoirs and pressed record.....pumped the brake pedal engine off and engine on. Disc brake resevoir had a big swirl each stroke. Drum resevoir only swirreled on the first pump and after a 30 second wait, and those rear resevoir swirls were tiny by comparison. I musta partly blocked the rear vent with my adjustment earlier. Popped the master off, moved the pin back into the booster about .010" and got out the prussian blue again.....no contact between pin and piston. Put the camera back on and pumped her a bunch engine off and engine on, big ol nasty swirls in both bowls! Supper time now, so, test drive tomorrow. Hoping to continue to sneak up on perfection.
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