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)?
So these aren’t low drag calipers on the front? A larger bore master would move more fluid, sooner, so you’ll feel it sooner. But if this system was working fine before you cooked the drums, I don’t think I’d change the master. I’d change the drums. Sent from my iPad using The H.A.M.B. mobile app
Can’t suggest anything for the rears with exception of making the correct with new shoes. With a 5-1 pedal you could dump the power booster unit, install a 7/8” MC or better yet a 3/4” and have great brakes without power. 4 or 6 piston calipers will make it even better. I have 4 piston and don’t need a vacuum assist.
I have lots of reservations in using a generic Bendix-type combo valve. Not just from a 'non-traditional' stand-point, but also from adding unnecessary complexity in a system. Especially in diagnostic purposes. But I'll bitch about that in a moment. When using these calipers, they are designed to be used with the 'quick-take-up' MCs that have a stepped bore. Usually a large 36mm piston to get the pistons/pads out to the rotor, and then a smaller 24mm bore that operates the brakes in a normal fashion. Piston seals on these calipers retract the pistons a bit to clear the rotors. This was done to increase mileage by reducing brake drag. Using these calipers without a quick-take-up MC can cause for unwanted long pedal as up to the first half of pedal stroke could be needed just to bring the pads in contact with the rotors before any actual braking is done. Such as... If you are using a non-takeup MC then the primary(disc) side is requiring more pedal(fluid) to push the pistons out. Since the MC circuits are hydraulically coupled you can get some strange pedal feel. 400°F is on the high side, but nothing unusual for hard stops. 600°F would be an issue for NAO linings. If you are running semi-metallic shoes they always work better when warm to hot. As long as you are not smelling brakes then you are fine. Since the temps dropped to a more normal temp of 200°F then it is not the drums are faulty, they are working fine. You need to look at the other half of the brake system. They are most likely getting hot under normal driving due to the front half of the system not working properly. Total system works in harmony, normal pedal, 200°F drum temp. Pedal gets long, light comes on, drums are doing most if not all the work, 400°F drum temp. Don't worry too much about drum temps, they can take the abuse, its the linings you need to verify you are not overheating. But they can be 100-300°F cooler than the drum temp. First, verify that your MC is plumbed correctly. Disc/Drum and Disc/Disc systems usually have the MC plumbed with the front(primary) to the rear(primary/closest to pedal) port of the MC. And the rear system is plumbed to the front(furthest from pedal) port of the MC. With the MC plumbed correctly to the Combo valve, the pressure will be equal between primary/secondary circuits. Pressure Differential Valve is the switch that will illuminate the brake warning lamp on the dash if there is a difference of pressure between the two circuits. light comes on. Pretty simple. It seems your system uses a self resetting system that allows re-centering if the pressure equalizes between front/rear. Metering Valve is to allow pressure to the front brakes after the rear brakes. Also known as a Delay valve. On disc/drum systems this is to allow the takeup of the rear drums then pressure to the front brakes for more even braking. This showed up in the 70's/80's with softly sprung vehicles. Otherwise under initial braking the car would nose dive quite a bit. With this metering system cars would not nose dive, but if rear drums fell out of adjustment(worn/jammed self adjusters) the brake pedal would become longer under normal usage. Some models will have a needle protrusion with a ball on the end, when bleeding brakes this needs to be pulled out to prevent the metering valve from trapping air. Usually a small spring steel U-clip with a slot is used to hold the valve open. Not to be confused with the differential valve dummy plug. Proportional valve, self explanatory, reduces pressure to the rear brakes for correct pressure under normal driving. With a combination of the metric calipers, delay 'metering' valve, and pad knockback, it is pretty easy to get the front circuit effective pressure very low. This would cause the differential valve to shunt and illuminate the brake warning lamp. As you now have a stack up of tolerances on the loose side that would upset the system. On the generic Bendix Combo valve. It's a handy junction block to bolt everything to, but it also has unnecessary and unqualified component for a custom setup. Metering valve needs to be used in conjunction with the quick take up MC and low drag 'metric' calipers. All three components need to be used. Brakes are a system. Preset Proportional valves can be more troublesome, or even dangerous, on a vehicle that was home built. Best to use an adjustable unit so you can fine tune in your own vehicles needs. Changing wheel sizes, lining material, compound type, and weight distribution all will affect how much pressure the rear brakes will need. IMO K.I.S.S. brake systems. Differential valve if you want a warning lamp. If you want to use a proportional valve, use an adjustable unit for fine tuning and better overall stopping. Even preset factory units are not even the best on a factory car, I've experienced reducing brake distance by 30-40' just from an adjustable prop valve tuned correctly. Nothing else changed. If we are to consider ourselves hot rodders, it can't just stop at the body or block. Brakes can also be hotrodded and tuned for better performance.
I had a corvette 1 inch master on my '34 roadster. It took a lot of pedal effort to stop the car and it would not lock the brakes. I went to a 7/8 master and t was better. I finally went with a 3/4 master and it stops on a dime with very little effort. So what Jimmy said worked for me as well.
Larger bore in the mastercylinder = more volume. That needed to push the pistons in some large diameter wheel cylinders out plus take up for the volume of fluid needed to move the shoes out to the drums. Smaller diameter bore mc = more psi but less volume. I got out of doing brakes or even teaching about them right at the time these quick take up master cylinders came out and even though I worked doing brakes in the mid 90s for a while most everything that hit that shop was oem stuff.
"...designed to be used with the 'quick-take-up' MCs... Part number or year/make/model? Also, since my pads ride in contact with the rotors at all times is it certain that they require a quick take up MC. I thought that would be for the ones where oads clear the rotors.
I worked on a 35 Ford with similar brake problems. It would stop but would not slide the tires. Not exactly confidence inspiring. Ended up swapping the booster from a 7 inch to an 8 inch unit. Much better braking. This is after swapping the master, the pads and shoes for brand new at the owners insisting.
The metric calipers have a smallish piston. The 'fix' used to be swapping them out for the calipers that come on full sized GM cars. But that requires new caliper brackets. Ran across these replacement metric calipers with larger piston bore the other day. These fit caliper brackets set up for the metric calipers. The Willwood polymatrix pads do cost more than el cheapo parts store types but are worth every penny and should last longer as well. Might be a large part of the answer to your braking issues...... https://www.southwestspeed.com/?sec=view_menu&cat=Hubs And Brakes&sub=Brake Calipers&ssub=!!!GM Metric&sssub=Steel Modified Calipers Left caliper, 2-3/4" piston...... https://www.southwestspeed.com/?sec=view_part&id=35739 Right caliper, 2-3/4" piston...... https://www.southwestspeed.com/?sec=view_part&id=35740 The good D154 pads (POLYMATRIX B PAD COMPOUND - set of 4) by Willwood - Part #WIL 15B-3998K...... https://www.southwestspeed.com/?sec=view_part&id=7529 The big bore replacement calipers are available from several sources, but make sure you use the ones made by Southwest Speed. They have the best rep with the circle track guys around here. Best regards, Harry P.S. >>> Also set up your pedal ratio for 6:1 as used in the older GM cars if what you have now is adjustable. I would suggest doing the big bore replacement calipers and Willwood linings along with 6:1 pedal ratio before replacing anything else. Your master cylinder may turn out to be fine when all is said and done. HB
Called mfgr. Of the suspension, calipers are non low drag. Verified with feeler gauge, only .005 would fit. Verified caliper pistons are about 2 3/8 " on tape measure. He recommended going to 7/8 or 15/16 master vice the 1 1/8 to have greater pressure. Thoughts/experience?
Going smaller on the master diameter will give more pressure but at the expense of increased pedal travel, and it's the pedal travel that's already concerning you! Going up on the master diameter will help (at the cost of less pressure) but 1" to 1 1/8" doesn't sound much but it is! You do the math (it's the area change, not the diameter, and the calc has a square in it!!). But up a little on the master and an 8" dual booster will probably get you where you want to be, strong brakes without excessive travel. Chris
Chris, those were my thoughts. Increase the bore for shorter stroke. Pedal is real easy now so even a third more leg will, I think, make if feel more like a newer car.
Reading all this and making the comments I did for what worked for me leads my thinking that a lot of hot rod brake systems are not well thought out. I have heard about stepped MC’s but didn’t know why or what calipers to use with them. Thanks Mike for the explanation. My other thoughts left me with the E-Bay$3-500 conversions are not we may think they are. Yes they could be fine along with buying a bunch of Rock Auto parts for different cars only getting the caliper mounts from a supplier. It it were not for knowledgeable people asking me what rear end wheel cylinder diameter I had, pedal ratio, etc I would be in a fix mode too for the front system. Every thing needs to work together.
I read your post. Check. replace shoes is a helpful suggestion. But, starting over to go full manual is not on our table. Given it's all factory type parts I'll try a new master, just gotta guess at larger or smaller. For 50 bucks or so its worth a try. Solution is likely to be simple I'm sure. 1 inch bore with factory (ish) stuff seemed right by all my research, but like a lot of projects, once the parts are together they kinda start telling you what wont work.
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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