BigTen Power Brake Questions/Advice needed.

Sharps40

Twelve Pointer
Here is what I have.

1937 Dodge 4 Door Sedan, 2850 lbs. 19 in vacuum. Disc/Drum 1" master with 7" dual diaphragm booster (fire wall mounted), new CPP PV2 valve, 71 Chevelle 12 bolt rear with rebuilt 9.5" factory drum brakes (new drum/shoes/cylinders) adjusted a tad snug, GM Metric front disc calipers on aftermarket independent suspension, all new hoses/lines, no leaks, no trapped air, pedal set for 5 to 1 ratio, push pin between booster and master is just a whisker off the piston and I get fluid movement in the master with the first 3/4 inch of stroke.

Situations:

Decent brakes but still a long stroke, will not lock up, don't inspire confidence for a panic stop as the all the braking is at the bottom of the stroke. It has never been a feel where the brake action starts working well at a half stroke, its all at the bottom. I think I can feel it bottom out on hard stops.

Things were well till I changed the axle bearings, when I reassembled the ebrake levers under the drum bound and the rear drums overheated (IR gun said 400 F on the drums immediately on parking the car), they are light straw to gold colored inside. (Light straw to gold color on steel indicates 450 to 550F ) After cooling off I got a brake warning light and pedal went to floor/barely stopped. Then it was fine. Then it did it again, then was fine. No fluid loss at all. So I have sanded rear shoes, rechecked master, bled all four 3 times/no air, readjusted rear shoes, etc.... I got the warning light again just at start up. Went out and all was well, lots of hard stops, no light but still not a confident system for panic stops, Final temp on the drums today after all the work was 200F.

Questions:

1. When the drums went to 400F/turned light straw/gold color, have I ruined them, the shoes and cylinders?

2. Assuming I maintain a 5 to 1 pedal ratio, would a larger 1 1/8 master cylinder provide me less stroke and perhaps better stopping feel (i.e. feel like brakes are working at mid stroke rather than always at the bottom of the stroke)?
 

bigten

Old Mossy Horns
Contributor
Rich, your drums are cast iron and I don't think that temp would create your issue.
I would suspect shoes prior to drums.
With the bore size being on the smaller side, it will not forward the same amount of fluid a larger bore will. With combination (disc/drum) brakes, for some reason I'm thinking they used a 1-1/16 or 1-1/8 bore master cylinder for adequate flow. That is a good question and I think you would be more satisfied with a larger volume flow.
 

Sharps40

Twelve Pointer
Im going to check the car today and if I get a light im going to try the S10/blazer master. Its got the ports on the driver side, sits level and has a 1.4" dia quick take up with a .95 piston. That should give me both short swing, more volume initially and plenty of pressure over all. Its probably what I needed to begin with but I thought i had the older imperial calipers.....turns out I don't!
 

Sharps40

Twelve Pointer
Thanks for all the help and commentary. Significant progress. I went to a larger bore dual master cylinder. Duralast NM1521 which has the 1 1/8" bore (replacing the 1" bore master that was on there), no reserve pressure valves and shallow piston cup. After bench bleeding, swapping back to and adjusting the short push rod pin and verifying the free play at the pedal was still about 1/4" at the linkage and bleeding the lines twice around (ran about another pint through it front and rear) brakes are MUCH improved overall and panic stops are straight line, quick and provides a feeling of confidence.

As expected, the pedal is harder now but no more so than my 08 Xterra or 03 Thunderbird. Of significant note I can feel the brakes engaging in the top 1/3 or so of the stroke rather than down at the bottom quarter. It feels more like all my daily drivers now, should make transition from daily driver to fun hot rod easier to manage.

At 5 to 1 ratio, plenty of good feel in that top 1/3 of action and easy to manage delicate braking. Overall I'm quite pleased with the change up and now have a spare 1" master cylinder for another project later on. (I guess it'll need larger piston calipers to "feel" right with a 1" bore and I'll have to keep that in mind or perhaps go to a larger diameter booster on that other project.)

A larger booster might help a bit more but for now I'll focus on getting those rear shoes checked and adjusted and perhaps play with the ratio at the pedal over the next several driving sessions.

With luck, I'll have this part of the project sorted soon and can move on to tweaking the pair of Rochester 2g carbs on the engine for best performance and driveability.
 
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