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Old 07-06-2008, 02:58 PM
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Master Cylinder Question!

I can't get pressure to the rear brakes! Here are the details...
>1971 Nova with an aftermarket front disk conversion.
>I replaced the rear member with a Ford 8.8 from a '95 Explorer that came with factory disks. I also replaced the calipers and pads with NAPA units (the Explorer rotors were in spec)
>The MC is a stock GM 2-reservoir "corvette" style (so called), and the booster is an unknown aftermarket unit.
>All the brake lines are clear and yes I do get fluid to all 4 corners.

Front brakes work just fine but not enough pressure to the rear to actuate the calipers.

1. The MC bench bleeds just fine with pressure from both front and rear orifices.
2. I have tried it with and without a proportioning valve (Wilwood adjustable). Makes no difference.
3. Have set the booster/MC push-pin to what I "think" are factory specs (3/32" as I remember...dialed just about all the way in).

I am pretty sure the problem is in the booster/MC setup, but I am very open to suggestions!!!

First, can anyone verify the booster/MC push-pin adjustment for a GM master cylinder? I'm wondering if maybe it isn't pushing the piston in far enough to actuate the rear brakes.

Thanks for your advice...I am REALLY getting frustrated!!

Dave

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Old 07-06-2008, 03:30 PM
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You you are getting fluid at the rears while bleeding but no stopping presure. I would get the proper fitting for the master to block off the front fluid port and see if you can get the rears to work with the fronts out of the picture. Did you eliminate the residual presure valve that the drum brakes required.
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Old 07-06-2008, 05:52 PM
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Quote:
Originally Posted by bluesman123
I can't get pressure to the rear brakes! Here are the details...

>I replaced the rear member with a Ford 8.8 from a '95 Explorer that came with factory disks. I also replaced the calipers and pads with NAPA units (the Explorer rotors were in spec)
>The MC is a stock GM 2-reservoir "corvette" style (so called), and the booster is an unknown aftermarket unit.

Front brakes work just fine but not enough pressure to the rear to actuate the calipers.

2. I have tried it with and without a proportioning valve (Wilwood adjustable). Makes no difference.
I have 4 wheel power assisted disc brakes and had similar problems. I have GM front and rear calipers. The rears are the ones that have the ratcheting emergency brake mechanism internally.

Here is what I found that should help you.

The rear calipers MUST be adjusted so that the pads drag slightly. This is because there will not be enough fluid coming from the master cylinder into the rear calipers to fully actuate them. The rear calipers retract much more than the front calipers (both Ford and GM) and thus require that they must be adjusted when replacing the pads (which you did). It is also very important that the emergency brake be used as this is what keeps the rear disc brake calipers properly adjusted.

Most of the power brake master cylinder aftermarket 'kits' come with a Corvette style master cylinder with a 1" bore. I replaced the one I originally used with one that had a 1 1/8" bore for additional volume.

Most of those 'kits' have a 6-1 pedal ratio which is fine if you are not using the power assist. I changed mine to a 4-1 ratio because with the 6-1 I lost half of the pedal travel.

Re-install the adjustable proportioning valve. You will need it to 'balance' the braking front to rear. If the rears lock up before the front in a panic stop, the vehicle will very quickly swap ends. NOT good! Most adjustable proportioning valves are fully open when the valve is screwed all the way down and reduce the pressure as much as 57% (advertised) when screwed all the way out. This seems backwards but that's what the instruction sheet stated.

Adjusting the pushrod from the pedal linkage to the booster should yield very little 'free' play. About 1/2" -1" at the pedal. There is a tool to adjust the pushrod coming from the booster to the master cylinder, but you can adjust it if necessary by measuring how far it sticks out of the booster and then measuring the depth of the pushrod recess in the master cylinder. A little tricky because of the radius. If you have the correct master cylinder for power assist, that recess will be fairly shallow. A master that is designed for use without the power assist will have a much deeper recess to prevent the pushrod from the pedal from falling out. I also added a simple return spring to assure that the pedal returned to 'home' when released.

If you have an under the floor master, you will need a 2 psi residual valve in the front line and because of the type of rear caliper being used, you will need a 10 psi residual in the rear. Usually disc brake calipers will only need the 2 psi but these need the 10 psi because of the additional amount that they can retract. Front calipers retract less than .010 in most cases. These rear retract .050-.060 unless adjusted as I mentioned above to limit how far they can retract.

I have attached some info that will help.
Attached Thumbnails
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Name:	pushrod_adjust_tool1.gif
Views:	91
Size:	12.8 KB
ID:	31006  
Attached Files
File Type: pdf partsTechBrakeT1008.pdf (215.4 KB, 81 views)
File Type: pdf pedalratiopdf.pdf (128.3 KB, 66 views)
File Type: doc residual pressure valves.doc (25.5 KB, 76 views)
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Old 07-06-2008, 06:13 PM
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Wow! Thanks Frisco! That's the first I have heard about the rear brake adjustment. Yes, the Exploder brakes do have an e-brake (drum) in the center. Yes, I do use the e-brake but I have not tried the adjustment you mentioned. For sure I am going to try that next.

Also, the MC is a 1" bore model. The next step would be to go with a 1 1/8" bore.

As far as I can tell from your diagram, the booster/MC rod is set correctly. ...and yes, it looks like the MC is designed for a booster as the cup is pretty shallow.

The Nova brake pedal comes with both a manual brake hole and a boosted brake hole. Right now I do have it in the proper hole.

We are having a heatwave this week (I live in the East Bay) so it may be a few days before I can try this out, but I will let you know what happens.

Dave
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Old 07-06-2008, 06:34 PM
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OK, dumb question: is your rear brake line free of internal obstruction, kinks, etc., from the MC all the way to the caliper connections? Sometimes it's the little things that trip you up. When you open the bleed valves at the rear calipers, does the fluid shoot out with good force? I'm trying to help narrow down the search.....I have GM disks at all 4 corners and 1" bore Corvette dual master cylinder, similar to yours. Your combo should work fine after we find the problem.
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Old 07-06-2008, 06:37 PM
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Quote:
Originally Posted by bluesman123
That's the first I have heard about the rear brake adjustment. Yes, the Exploder brakes do have an e-brake (drum) in the center. Yes, I do use the e-brake but I have not tried the adjustment you mentioned. For sure I am going to try that next.
Since your rear disc brakes use a drum style emergency brake the initial adjustment for the rear calipers may be different than what I described. An easy way to check, is to apply the brakes. Then release the brake pedal and using feeler gauges, see how much clearance there is between the rotor and the brake pad. There should be about .005-.010. If greater than that, then the calipers are retracting too much and will need to be adjusted to get the rear brakes to work properly. Do this before replacing what you have. Please post back your results when you have the opportunity as it will surely help others.
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Old 07-06-2008, 10:03 PM
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Yeah Antnyl that's the problem. Not much pressure when I bleed the rear brakes. Yes, the lines are all clear and no kinks. That's what made me think it is a problem between the MC and the booster. I wish I had a pressure gauge I could put directly onto the MC port.

Frisco, there is definitely too much clearance between the pads and the rotor. The pads don't even grab enough to polish the rust off the rotors! There is just a slight amount of buffing on the rotors...that's all.

T-Bucket gave me an interesting idea. Wonder if I could jigger the rear lines onto the front port of the MC? I may try to see how far I can get with that?

Another possibility is to go by Pick-n-Pull and yank a MC off another GM and see what that does!

Dave
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Old 07-07-2008, 07:31 AM
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Quote:
Originally Posted by Frisco
Since your rear disc brakes use a drum style emergency brake the initial adjustment for the rear calipers may be different than what I described. An easy way to check, is to apply the brakes. Then release the brake pedal and using feeler gauges, see how much clearance there is between the rotor and the brake pad. There should be about .005-.010. If greater than that, then the calipers are retracting too much and will need to be adjusted to get the rear brakes to work properly. Do this before replacing what you have. Please post back your results when you have the opportunity as it will surely help others.
Frisco,

What's the procedure for adjusting the calipers? I bolted mine up without adjusting anything, wasn't aware that they could be adjusted.

Dave,

If the lines are clear then it def sounds like a problem with the MC. The rear plunger seal might be shot. You might be able to get a rebuild kit. Is it new?

Antny
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Old 07-07-2008, 07:50 AM
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Quote:
Originally Posted by AntnyL
Frisco,

What's the procedure for adjusting the calipers? I bolted mine up without adjusting anything, wasn't aware that they could be adjusted.
For GM read the attachment above. It is a pdf file named partsTechBrakeT1008.

I believe the Ford calipers (and some other makes) need to be manually screwed in to make enough room when replacing the pads. A tool for this is available at any auto parts store (even places like Advance Auto and Pep Boys). They then have to be trial fit over the rotors and the piston is then un-screwed some and trial fit again until the correct initial clearance is achieved.

In either case, the rear calipers must be adjusted correctly before they will function and a decent brake pedal is achieved.

The photo below is one type of caliper adjusting tool. This LINK is for a better more sophisticated caliper adjusting tool.
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Last edited by Frisco; 07-07-2008 at 07:57 AM.
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Old 07-07-2008, 08:00 AM
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Quote:
Originally Posted by bluesman123
I can't get pressure to the rear brakes! Here are the details...
Are you using a factory-style distribution block with the differential pressure valve (that triggers the BRAKE light on the dash if you lose pressure in one side)? If so, it is common for the valve to slide all the way to one side when bleeding the brakes, which may cause exactly the problem you describe.
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Old 07-07-2008, 10:07 AM
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I think I'm really getting some great advice here! I wish I had asked you guys earlier and maybe saved me some aggro!

Thanks Joe...good suggestion. I'm not using a distribution block. I've thought about using one to get the warning light but first things first! I just have an adjustable proportioning (dang, that's hard to spell! valve. I have tried it both with and without the valve.

Frisco, I never tried adjusting the rear calipers. I've never replaced Ford calipers and didn't know you needed to do that. I will check it out and see if it makes a difference.

I found these instructions on the web. The pics do look like the same brakes I have, so I am hoping it is similar.
http://probefaq.org/service/rear-pads.html

First, howsomever, it has to cool off a bit. Over 100 every day this week and I'm doing this in the driveway!

Dave
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Old 07-07-2008, 12:06 PM
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Here are two more pdf files about servicing the Ford disc brake calipers and how to adjust them. One is for early Ford disc brakes and the other is for later models.

Either the one you found for the Ford Probe or possibly one of these two should give you the info you need.
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File Type: pdf partsTechBrakeT1024.pdf (91.7 KB, 91 views)
File Type: pdf partsTechBrakeT1025.pdf (45.7 KB, 85 views)
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Old 07-08-2008, 10:52 PM
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Quote:
Originally Posted by Frisco
Most of the power brake master cylinder aftermarket 'kits' come with a Corvette style master cylinder with a 1" bore. I replaced the one I originally used with one that had a 1 1/8" bore for additional volume.
A larger bore master will push more volume to the calipers, but it will do it with less pressure. I think my first step here would be to invest in a brake pressure gauge to see exactly what's going on.
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Old 07-09-2008, 03:22 AM
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no offence, frisco but you DO NOT adjust the e-brake to "tighten up" explorer calipers.
the ebrake shoes/drum are on the inside of the rotor.
the calipers work like the front, no adjustment possible.

how does the pedal "feel"?
does it feel rock hard or does it seem to have some assist?
(with the engine running)
if you go to a 1 1/8" m/c the pedal will get harder...but have less travel and less pressure.
smaller bore will have more pressure and more travel.
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Old 07-09-2008, 08:07 AM
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Quote:
Originally Posted by tomslik
no offence, frisco but you DO NOT adjust the e-brake to "tighten up" explorer calipers.
the ebrake shoes/drum are on the inside of the rotor.
the calipers work like the front, no adjustment possible.

how does the pedal "feel"?
does it feel rock hard or does it seem to have some assist?
(with the engine running)
if you go to a 1 1/8" m/c the pedal will get harder...but have less travel and less pressure.
smaller bore will have more pressure and more travel.
No offense taken.

You evidently did not read what I posted above. Here is what I stated in post #6.

Quote:
Originally Posted by Frisco
Since your rear disc brakes use a drum style emergency brake the initial adjustment for the rear calipers may be different than what I described. An easy way to check, is to apply the brakes. Then release the brake pedal and using feeler gauges, see how much clearance there is between the rotor and the brake pad. There should be about .005-.010. If greater than that, then the calipers are retracting too much and will need to be adjusted to get the rear brakes to work properly. Do this before replacing what you have.
I also attached several pdf files explaining different methods of adjusting the rear calipers for early and late Ford rear disc brakes.

You and ceh383 are quite correct in stating that going to a larger bore will yield less pressure and more volume. Because of how far the rear calipers retract the additional volume is needed. He has power assist and will not even notice this reduction in pressure. If he was not using the power assist then he definitely would be better off using the 1" bore master as well as a 6:1 or 7:1 pedal ratio.
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