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    Results 1 to 12 of 12
    1. #1
      Join Date
      Aug 2011
      Location
      Sevierville, TN
      Posts
      524
      Country Flag: United States

      Tell-tale sign of a bad MC

      OK, I have a 68 C10. It's equipped with TCI Engineering suspension with Wilwood 13" 6 piston brakes with Polymatrix E compound pads up front, CaptainFab rear disc brackets with 12" Centric drilled/slotted rotors and GM truck D52 floating calipers with stock replacement pads from NAPA on the rear. All this is being controlled(supposedly) by a Wilwood 15/16" tandem master cylinder/ adjustable proportioning valve kit.

      Upon first installation and driving, the pedal has always been spongy, and during autocrossing, the brakes seem to have a delayed effect, and/or pad knockback since the pedal just sinks to the floor until pumped a time or two. After research, I installed both [/url=http://store.drpperformance.com/bearing-spacer-for-gm-impala-spindle-adjustable-aluminum/]DRP bearing preload spacers[/url] to reduce any spindle deflection, and Wilwood 2lb. residual valves hoping to cure these issues.

      However, even after these parts installed, the pedal still feels spongy, and almost feels like there's a 2-part pedal between front and rear brakes. I know this sounds odd, but it's like about halfway through pedal travel, there's a sudden stiffness that I can feel predominantly while pumping the pedal while bleeding the FRONT calipers only. As typical with brake bleeding, when the front caliper bleeder screw is opened, the lower half of the pedal sinks to the floor, BUT there's NO pedal drop when bleeding the rear calipers. There's also very little fluid pressure from the rear calipers during bleeding. But under hard braking, the rear brakes want to lock up before the front brakes even feel like they're starting to grab. I've been adjusting the rear valve to cure the rear lockup, but why won't fronts grab like they should?

      What kind of symptoms does a Wilwood tandem MC show when bad? Also, if my wording makes no sense, I can try to explain better, somehow.
      Matt Kenner

      68 C10 stepside

      If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

    2. #2
      Join Date
      Sep 2010
      Location
      Beach Park IL
      Posts
      2,838
      Country Flag: United States
      Do the math on your piston area and I think you will see the problem with your spongy pedal and tendency for the rears to lock up. I think the easiest way out would be to swap your stock rear calipers for a set of the Wilwood D52 with the _small_ pistons. https://wilwood.com/Calipers/Caliper...0Dual%20Piston

      Regarding the "two part pedal" when bleeding, that is exactly how it should feel when the fronts are bled and the rears aren't. You always want to bleed the end of the car that is connected to master cylinder port closest to the pedal first. With the fronts already done, you cant get a full stroke in the master to bleed the rears. If you crack a front bleeder and hook a hose to it and leave the other end of the hose submerged in a bottle of brake fluid you will be able to bleed the rear brakes as normal.
      Donny

      Support your local hot rod shop!

    3. #3
      Join Date
      Aug 2011
      Location
      Sevierville, TN
      Posts
      524
      Country Flag: United States
      Quote Originally Posted by dontlifttoshift View Post
      Do the math on your piston area and I think you will see the problem with your spongy pedal and tendency for the rears to lock up. I think the easiest way out would be to swap your stock rear calipers for a set of the Wilwood D52 with the _small_ pistons. https://wilwood.com/Calipers/Caliper...0Dual%20Piston

      Regarding the "two part pedal" when bleeding, that is exactly how it should feel when the fronts are bled and the rears aren't. You always want to bleed the end of the car that is connected to master cylinder port closest to the pedal first. With the fronts already done, you cant get a full stroke in the master to bleed the rears. If you crack a front bleeder and hook a hose to it and leave the other end of the hose submerged in a bottle of brake fluid you will be able to bleed the rear brakes as normal.
      So, you're saying the GM calipers are too strong for the rear?

      I've never tried bleeding the brakes with the fronts still open. I've also always been taught to bleed farthest(right rear) to nearest(left front) in relation to the MC. I actually prefer the gravity bleed method, but we were pressed for time since I was at a friends shop and it was getting late.
      Matt Kenner

      68 C10 stepside

      If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

    4. #4
      Join Date
      Aug 2011
      Location
      Sevierville, TN
      Posts
      524
      Country Flag: United States
      Can someone explain piston area, and what affect it has on how a caliper works?

      My 6 piston Wilwoods have a 4.04 Total piston area. The Wilwood D52 calipers with 2" pistons have a 6.28" piston area. But the stock 2 15/16"single bore GM D52 calipers have a 6.77" total piston area.

      I was never great at math or physics, and now my head hurts.

      FWIW, my truck stopped better with stock 12" front brakes with Hawk HPS pads and stock drum brakes than it does now with all this expensive Wilwood stuff.
      Matt Kenner

      68 C10 stepside

      If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

    5. #5
      Join Date
      Oct 2009
      Location
      CT.
      Posts
      738
      Doe sit matter on a Wilwood m/c which port goes front or rear? I know a stock GM style the rear larger port should go to the front. But the Wilwood has the same size reservoir front and back.



      Glenn

      1955 Chevy BelAir
      1951 Chevy 3100
      1987 Chevy Silverado

      My last project....
      https://www.pro-touring.com/threads/...my-72-Maverick!!

    6. #6
      Join Date
      Sep 2010
      Location
      Beach Park IL
      Posts
      2,838
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      Start here.

      https://www.pro-touring.com/threads/...Tobin-of-KORE3

      You just laid it out though. Your stock calipers had a larger piston area than your new ones. A larger piston will create more clamp load at the caliper than a smaller piston will for the same line pressure.

      Your rear piston area is too large even for your stock front calipers, but your new front calipers are even smaller so your brake bias is shifted to the rear. That is why the rears want to lock up all the time.

      On top of that, your master cylinder is probably about right for the front brakes but there is no way it can move enough volume for the rear brakes......because they are huge.

      I don't like those calipers on the rear of anything, especially a truck. The tiny Wilwoods I linked above with the 2.46" piston area will probably cure most of your issues.
      Donny

      Support your local hot rod shop!

    7. #7
      Join Date
      Aug 2011
      Location
      Sevierville, TN
      Posts
      524
      Country Flag: United States
      Quote Originally Posted by dontlifttoshift View Post
      Start here.

      https://www.pro-touring.com/threads/...Tobin-of-KORE3

      You just laid it out though. Your stock calipers had a larger piston area than your new ones. A larger piston will create more clamp load at the caliper than a smaller piston will for the same line pressure.

      Your rear piston area is too large even for your stock front calipers, but your new front calipers are even smaller so your brake bias is shifted to the rear. That is why the rears want to lock up all the time.

      On top of that, your master cylinder is probably about right for the front brakes but there is no way it can move enough volume for the rear brakes......because they are huge.

      I don't like those calipers on the rear of anything, especially a truck. The tiny Wilwoods I linked above with the 2.46" piston area will probably cure most of your issues.
      I'll see if I can cipher through all this! lol
      Matt Kenner

      68 C10 stepside

      If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

    8. #8
      Join Date
      Aug 2011
      Location
      Sevierville, TN
      Posts
      524
      Country Flag: United States
      OK, I installed the residual valves a few days before leaving for vacation and didn't get to spend much more time on the truck, but before I left, I noticed there was a slight leak on the rear R/V and I didn't have time to mess with it. We got back from vacation Saturday night, so on Sunday, I wanted to start on it. I had to go buy some flare nut wrenches (yes, I had some, but they didn't quite fit the fitting correctly) to tighten the fittings for the valve. After tightening and no more leaks, I gravity bled the brakes. I got a few air bubbles out, but didn't let it bleed long enough to make for certain there was no more air. I went for a test drive, and I've got the rears adjusted down far enough that they don't lock down as easy, but I'm still going to upgrade to the Wilwood D52s soon.

      My problem is still pad knockback and an inconsistent pedal (which the pedal could be because the rear calipers.) The residual valves seem to be helping some as the pedal pumps up quicker than before. I used to have to pump about 2-3 times, and now 1-2 pumps seem to get the pedal pressure back. I'm currently looking into the caliper piston springs to hopefully cure the rest of the knockback issue.

      I'm also gonna drive the truck a few days this week and get some heat cycles through the brake system and re-bleed again before Bill's event this weekend.
      Matt Kenner

      68 C10 stepside

      If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

    9. #9
      Join Date
      Aug 2010
      Location
      now In Dandridge, Tn.
      Posts
      1,301
      Country Flag: United States
      Here's my $.02. I have seen Matt's truck in action, and had a good look at the suspension set-up. I am not too familiar with the rear brake bracket. But, if the caliper mount is not flexing, there should be almost no 'knock back' due to the floating caliper. - Although, many aftermarket mounts for D52 and D154 style calipers do not have a load arm to keep the caliper square. In this situation the caliper will tend to 'twist' under braking, and then square up as the pressure drops. This can cause two problems, 1) a feeling of 'knock-back' and 2) uneven pad wear, resulting in a degrading brake performance over time. (this is common in many MII hot rods). I agree that the rear is over powering the front, but not sure how much. My calcs come up with 4126 ft / 6247 rr. (this is off a chart I made, using a tandem Master 5/8" bore, so numbers are higher, but % is the same). So, if you have 4126 up front, and that should be 70% of braking, you would want 1768 in the rear. Wilwood has two other D52 (both smaller piston area than the GM) one has 6.28 sqin, the other has 2.46 sqin. If I sub in the 2.46 caliper into my chart, I get a rear brake potential of 2248. still a little high, but a LOT closer. Now, if you were to swap front calipers, from the #120-11781 with 4.04 sqin, to the #120-14540 with 4.86 sqin, that would boost the front number from 4126 to 4951 (these numbers are in FtLbs of torque available) Now, a balance of 4951 ft / 2248 rr (7199 total) would set the balance at 68.8% ft. Which, if everything was working correctly would be great.
      SO, my proposed solution includes four new calipers. That's $$$$ for a "tune", and that sucks. (getting my soap box out) Everyone needs to understand that a lot of this is crap. We DON'T drive around in daily traffic using 100 lbs of foot pressure on the pedal. Heck, most of us never press that hard on a track day. 200 TW tires WON'T hold up to it. We are spoiled by our late model drivers that require 20 - 40 lbs for a full stop. My old charts and spreadsheets use 100 lbs and I'm to lazy to change them, BUT I know this is FALSE, and when I work out a new system, I use 60 lbs. ALSO, 5-1 or 5.5-1 pedal ratios (that come on the fancy $$$ pedal kits) are not for a drivers. Not even spirited drivers. That's for RACE cars. Like 3/8 mile oval Modifieds. If you want to be able to modulate the brakes, like in a real car, think 6.5 - 1 ratios. Next, for temp reasons, most aftermarket kits are "under-calipered" -Yes, I made up that term. A stock GM singe piston caliper has 6.775 sqin of piston area. On a 12" rotor, this is pretty awesome (90's Impala cop car) So, if you're going to spend $2k on an "Upgrade" the new kit should at LEAST come close to this. (not many do). So, before you order a kit, look up the calipers they use use, and see if there is a similar caliper with larger pistons that can be substituted in. Yep, it's a lot of math. but that's how it works. Why do they do this? IDK. But that's how it is. ANd, if we understand that, then we can work around it to get the results we want. (OK, off the box) Personally I would like to see Matt's truck put out it's peak potential performance. Just to see how it stacks up. He's put a LOT of time and $$ in it, and it SHOULD pay off. Hope you get it sorted. Let me know if I can help.
      Last edited by RobNoLimit; 06-20-2018 at 06:59 AM.

    10. #10
      Join Date
      Aug 2011
      Location
      Sevierville, TN
      Posts
      524
      Country Flag: United States
      Thanks for the info, Rob! Now my head hurts more! :D

      What about the D154 rear brake kit?

      The D154 caliper shows 1.98 piston area. Where would that put the balance at?

      I talked to a guy last night that's ran dirt track cars for years. He said he uses offset bore master cylinders. Usually 3/4" bore for the front and 7/8" for the rear. I'm not sure I'd want to go that route yet, but he threw that in as an option.
      Matt Kenner

      68 C10 stepside

      If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

    11. #11
      Join Date
      Aug 2010
      Location
      now In Dandridge, Tn.
      Posts
      1,301
      Country Flag: United States
      That would be a pretty close match to the current front brake. Looks like it would be 71% ft. Just remember, you are playing the tuning game to the "Less brakes" side. I always try to tune to the "More brakes" side. There are some "stepped bore" masters out there (one dual chambered master with two bore diameters). Like say, 1" and 7/8". I used to play around with some of these. Think, Expensive, Hard to get, Hard to get parts for..... Many racers use a "tandem" master cyl set up. Two masters driven off of the same pedal. Usually with a "balance bar" for tuning. I do this a lot with manual brakes. I usually run a 5/8" front, and 3/4" rear.

    12. #12
      Join Date
      Aug 2011
      Location
      Sevierville, TN
      Posts
      524
      Country Flag: United States
      Yeah, Rob, I never thought I'd be considering "smaller" brakes.

      I can't honestly say I haven't considered putting stock brakes back on it.

      Matt Kenner

      68 C10 stepside

      If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue






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