Explorer rear discs: not working at all! Thoughts???

[chuzie]

Update.

Turned rotors.
Installed new calipers.

NO F'ing BRAKES!

Any thoughts?

 

[Apogee]

Good job on eliminating at least one issue with the rotor/flange interference...a few more and you'll be done.

When you say that you have a given pressure at the caliper, 200 psi, 750 psi, etc, where exactly are you measuring that value and what are you using to measure it?

 

[chuzie]

I am measuring at the bleeder port in the caliper using SSBC A1704 Brake Pressure Gauge Kit

 

[chuzie]

Had a great conversation with Apogee this afternoon. He was kind enough to donate his personal time and try to talk me off the ledge. Thanks bro, I really appreciate that.

The last remaining hope was to perform a leak down check and determine if a fitting or other leak may be at fault.

I secured the pedal as best I could for 30 minutes and they system maintained a pressure of 600psi the entire time.

Anyone want to buy an explorer disc brake kit real cheap? I think I am done. There is literally nothing else.

 

[chuzie]

Interesting tidbit of info today from Wild Horses. I had been measuring 600psi on this system. After researching, Justin found the booster (8" dual diaphragm) + MC (1.125" bore) combo they sold me should yield 1,400psi.

I wonder if that could be the issue?

That would change the braking torque from 6,292 in-lbs to 14,682 in-lbs.

Need to call them back tomorrow and see if they can find out the minimum required engine vacuum required to operate that booster.

 

[carmi]

This has been a very interesting thread! I am having similar problems with a caddy rear setup. My front brakes work great but the rears never fully engage. While I believe my problems are more related to the caddy caliper style (integrated e-brake) I do see a strong resemblance. My observation would be if the front brakes are engaging sooner than the rears than the pressure in the rear would never build unless you were to lose pressure on the fronts. I guess what I would suggest is to open a bleeder on the front while checking the pressure in the rears. This would allow full m/c piston movement and therefore allow full pressure to the rears. On my system, When I bleed the fronts the pedal will go all the way to the floor. When I bleed the rears, the pedal does not go all the way to the floor. My conclusion (at least for my situation) is that I almost need a stepped piston m/c to force more fluid to go to the rears. I did install a 10# (yes 10#) residual valve to the rear to attempt to keep the rear pistons from retracting so much but it wasn't the miracle cure I had hoped for. Ultimately I think the hodgepodge of brake parts is creating the issues. Have you checked the piston size of the front calipers vs the rears? The 600# pressure reading was at the rear caliper? Is it also 600#at the front? Are you running a combination valve or just running straight from the m/c to the calipers? For comparison I am using a disc/disc 1.125 corvette m/c with Chevy front calipers and caddy rears. In addition I have a Chevy 2500 hydro boost with a wildwood proportioning valve on the rear circuit but it is fully open. In short, I think my (and possibly your) front caliper piston bore is to small in relation to the rear piston size. Or it could just simply be be air in the lines. lol. I feel your pain and will be watching this thread to see what you come up with. Brakes suck!

 

[broncnaz]

Interesting tidbit of info today from Wild Horses. I had been measuring 600psi on this system. After researching, Justin found the booster (8" dual diaphragm) + MC (1.125" bore) combo they sold me should yield 1,400psi.

I wonder if that could be the issue?

That would change the braking torque from 6,292 in-lbs to 14,682 in-lbs.

Need to call them back tomorrow and see if they can find out the minimum required engine vacuum required to operate that booster.

Its commonly recommended that 18in of vacuum is needed for proper booster operation. Its said that lower vacuum will result in a harder pedal which stands to reason because the booster is not assisting as well at lower vacuum. I believe 14-16in is usually the lowest recommended before needing a vacuum pump. Also smaller boosters like 7in ones give less pressure.
Overall Id suggest dropping the MC size back down to 1in. that will increase the pressure you may get slightly more pedal travel but that could help the rear braking as well.
Its either that or do what many do to cover up there ill working brakes swap in H boost.

 

[broncnaz]

Ultimately I think the hodgepodge of brake parts is creating the issues. Have you checked the piston size of the front calipers vs the rears? The 600# pressure reading was at the rear caliper? Is it also 600#at the front? Are you running a combination valve or just running straight from the m/c to the calipers? For comparison I am using a disc/disc 1.125 corvette m/c with Chevy front calipers and caddy rears. In addition I have a Chevy 2500 hydro boost with a wildwood proportioning valve on the rear circuit but it is fully open. In short, I think my (and possibly your) front caliper piston bore is to small in relation to the rear piston size. Or it could just simply be be air in the lines. lol. I feel your pain and will be watching this thread to see what you come up with. Brakes suck!

He's using T bird calipers in the front which are about as big as you can get total of 7.51 sq in of piston and his rears are explorer which are fairly small with about 2.75 sq in of piston
vs your caddy calipers if there the large ones with 3.5 sq in and Gm calipers with 6.78 sq in

Air in the lines is really not a issue he's been thru that over and over.

 

[chuzie]

I will try popping a bleeder while measuring pressure at another caliper and report back.

WH seems to think 13 is enough but I too think it is low. Hoping their booster mfgr sheds some light on that.

I know I can increase timing slightly to get more vacuum but I don't think it will be significant especially with EFI.

Maybe I can find a pump and test the theory.

 

[chuzie]

Carmi, no valves in the system. All direct connec to CPP corvette MC.

Other folks make this system work with the same parts I have. Gotta find the pressure variable first.

 

[70_Steve]

Maybe I can find a pump and test the theory.

If not, make a vacuum aspirator out of some tubing/pipe fittings. I've made them before and you can get better than 20"hg out of them.

 

[chuzie]

WH seems to think 13 is enough but I too think it is low. Hoping their booster mfgr sheds some light on that.

Turns out 18" vac is required for optimal use.

 

[chuzie]

I guess what I would suggest is to open a bleeder on the front while checking the pressure in the rears. This would allow full m/c piston movement and therefore allow full pressure to the rears. On my system, When I bleed the fronts the pedal will go all the way to the floor. When I bleed the rears, the pedal does not go all the way to the floor.

When I crack the front bleeder (engine running), I can effortlessly push the pedal to the floor. Same thing with the rear. Pressure still at 600psi

Its commonly recommended that 18in of vacuum is needed for proper booster operation. Its said that lower vacuum will result in a harder pedal which stands to reason because the booster is not assisting as well at lower vacuum. I believe 14-16in is usually the lowest recommended before needing a vacuum pump.

You are correct. The booster mfgr suggests 18". I measured my vac again today and it was 13". Bumped the timing to 12BTC and got it up to 16" but no change in pressure or pedal.

If not, make a vacuum aspirator out of some tubing/pipe fittings. I've made them before and you can get better than 20"hg out of them.

How do I do that? I bought an electric vac pump but I tested it at 12". $80 piece of junk!

Ended up connecting my vac tester to booster and pumped, no shit, about 10,000 times to get to 20" vac. Still at 600psi.

I still need to find out why the engine has such poor vacuum but I am confident my engine vacuum is not the culprit here. There has to be something going on with this booster or MC.

I agree going to a 1" bore will help my pressure but since I should be seeing 1,400psi with this setup, I think changing to a different bore will yield similar results.

Does anyone think it is weird that my pedal goes to the floor effortlessly when a front or rear bleeder is open?

 

[70_Steve]

How do I do that? I bought an electric vac pump but I tested it at 12". $80 piece of junk!

Might be easier just to buy one. $14 on Amazon, and it pulls to 28 1/2"hg

http://www.amazon.com/Nalgene-6140-0010-Aspirator-Polypropylene-Threading/dp/B003OBYNBO

 

[broncnaz]

1 1/8"bore mc and a 6-1 pedal ratio will only get you about 450 PSI with 75 lbs on the pedal. So your 600 psi reading is about right but I dont think your booster is helping any.
A 1in bore mc with the 6-1 pedal no booster will net you around 600 psi with 75 lbs of pedal pressure.
While I have no hard facts a power booster should more or less double the force applied to the MC. I dont think your getting any boost.
If your MC goes to the floor when 1 bleeder is open something is wrong with the MC. Namely a seal is bypassing. Yes i realize you've had several MC's with the same results. You have a ssue somewhere and I dont think a different rear brake setup is going to solve it.

 

[chuzie]

Agreed 100%. I never had a reason to apply brakes and measure pressure with engine off and check valve disconnected, but your post inclined me to do so. Same 600psi; big surprise there. Gotta call WH and figure something out. Gonna need booster #2 and MC #3. I surely have no favor with the brake gods.

 

[DirtDonk]

One thing you could do quickly (assuming a good donor vehicle is handy) is to connect a long hose from a known high-vacuum engine and run it to your booster. Then check pressure at the gauge.

Obviously, this method won't allow you to test the brakes while driving, but at least you can see if higher vacuum raises the working pressure.

Back to the discussion of axle shafts... I don't remember seeing what axle shafts you're actually using. Are they the stock '77 model shafts? If so, how is this compatible with the Explorer brackets just being bolted straight to the axle housing end? I thought to do that required the '74/'75 axle shaft flange offset.
Maybe it's not a big enough difference to cause any trouble, and was only an issue with the bolt hole pattern of the mounting bracket itself, but the thought popped into my head awhile ago so thought I'd bring it up at least.

Is this a custom brake setup put together by you from the very beginning? Did you have to modify the bracket at all to get it to fit?

Paul

 

[chuzie]

My rig is a 77 bit I'm running 75 rear axle. I will hook it up to my honda but im pretty convinced with the 18" I had on it yesterday.

Going to throw a different 1-1/8mc on it this weekend and see how it reacts.

 

[DirtDonk]

Yeah, with 18 you should've been good. But I suppose it doesn't hurt to try (since it's relatively convenient) with a little more just to see if you get a different pressure reading with 20 to 22 or whatever the Honda has.

Paul

 

[chuzie]

Swapped MC and still no change in pressure. Measured 600 @ MC. Interestingly, when I crack the rear bleeder the pedal goes to the floor but not when I crack the front. I suppose this is an improvement from the other MC where the pedal went to the floor no matter what bleeder was cracked open. What are the chances 5 of the last 5 MCs I've been through are bad? There has to be something else here.

Honda runs @ 18" so no dice there.