Disk brake questions

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Topic author
Stelzer
Posts: 5
Joined: Sat Jul 10, 2021 10:28 pm

Disk brake questions

Post by Stelzer »

Hey guys I have a 72 j4000, I have swapped in a 44 front and 14bolt rear with 4 wheel disk brakes. My question is if anyone has any insight on what I need to do to get good braking. Have have a new booster installed with a 4 wheel disk master and still have no braking pressure. Any insight would be greatly appreciated

rocklaurence
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Re: Disk brake questions

Post by rocklaurence »

I have a few questions. Does pressure increase as you pump it? Second, when you bleed the calipers is there good flow? Ive had rust in the steel lines restrict flow and cause issues. Most likely its air in the lines.
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Yeller
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Location: Rogers County Oklahoma

Re: Disk brake questions

Post by Yeller »

What calipers and master cylinder are you running? If they are matched for size volume often is an issue and are difficult to bleed pedal stroke is long and pressure is low. For instance 3” bore calipers with a 1” and often 1-1/8” bore master cylinder will be difficult to bleed and will take most all of the stroke from the master cylinder to apply the brakes making it difficult to bleed and if your pedal assembly does not allow for full stroking of the master may not have any brakes at all. Not on my J truck but I have a set up with 3” bore rear calipers and a 3-1/2” fronts and had to go to 1-5/16” bore master to have decent pedal, which made me go to a larger booster to develop enough pressure to have good brakes, when applied the brakes had good pedal but was just hard.
The bus I ride is so short it is a yellow Smart Car full of squirrels, monkeys and clowns.

1970 J2500 Resto Mod
https://www.fsjnetwork.com/forum/viewt ... 12&t=21395

1974 Bronco “Broncno”
https://classicbroncos.com/forums/threa ... st-3411909

itselliott
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Re: Disk brake questions

Post by itselliott »

I had an issue similar. Drove me nuts. Then I realized that the front calipers bleeder valve was actually below the top of the piston. Bled the system with the calipers off and rotated so that the valve was above the chamber .....all better after that.
This was a CJ with the "OEM Big Brake" calipers that were only used for a couple of years by AMC. They were a factory option.

Topic author
Stelzer
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Joined: Sat Jul 10, 2021 10:28 pm

Re: Disk brake questions

Post by Stelzer »

Thanks for the reply’s
Pressure does not increase as I pump it. I just flat out have not real pedal at all. I have blown out all the steel line’s many times over the course of the build thinking that was a culprit, I have bleed several times with a power bleeder and manually bleeding it. The bore on the master is a 1” not sure on the caliper bore size. My booster is just a new factory style. Kinda sounds like I may be having the same issue going on and need to go to a bigger booster and bigger bore master.

sonoraed
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Re: Disk brake questions

Post by sonoraed »

Proportioning valve ? Had problem when I went to 4W disc brakes with old disc/ drum valve , bought all disc valve from bj’s , able to bleed all around and have great brakes


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Yeller
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Location: Rogers County Oklahoma

Re: Disk brake questions

Post by Yeller »

The 1”bore master cylinder is more than likely the culprit. Especially if you have the standard GM calipers on the rear, it will not move enough fluid to apply the brakes, or feel like it is anyway. I’ve put manual, non-power, brake systems together that you could push the pedal all the way to the floor with little resistance by hand that stopped well but we’re terrible scary to drive.

Pull a caliper off each axle and measure the pistons. From there we can help point you to the correct size master cylinder. More than likely if you did a stock single diaphragm booster you are going to want more, but that won’t keep you from making a good pedal.
The bus I ride is so short it is a yellow Smart Car full of squirrels, monkeys and clowns.

1970 J2500 Resto Mod
https://www.fsjnetwork.com/forum/viewt ... 12&t=21395

1974 Bronco “Broncno”
https://classicbroncos.com/forums/threa ... st-3411909

Topic author
Stelzer
Posts: 5
Joined: Sat Jul 10, 2021 10:28 pm

Re: Disk brake questions

Post by Stelzer »

Thank you I will try and get some measurements this week! Thanks for the info!!

Topic author
Stelzer
Posts: 5
Joined: Sat Jul 10, 2021 10:28 pm

Re: Disk brake questions

Post by Stelzer »

Looks like my calipers are 3 5/32 bore size
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Yeller
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Location: Rogers County Oklahoma

Re: Disk brake questions

Post by Yeller »

With the massive JD7 calipers you need a 1-1/4” bore master cylinder. I usually use 85 Chevy k30 for reference at the parts store, there are 2 versions, one with vacuum booster and one with hydroboost. The vacuum unit has a bored out section for the push rod and the hydroboost is flush. If you want a new unit, rockauto does stock a new AC Delco unit, I’ve only found reman from parts stores. You can also use a newer unit with a plastic reservoir but I haven’t done homework on that.

As for a larger booster there are a few options, none of them are 100% bolt in but not terrible. The easiest is a booster for 1980 c3 corvette, new (not reman) units are available for under $100. I went the slightly more complicated route and did hydroboost. Both of those options use the hydroboost style master cylinder. I have done the corvette booster on several vehicles, it’s a little smaller than something off a truck and will fit on the space easily.
The bus I ride is so short it is a yellow Smart Car full of squirrels, monkeys and clowns.

1970 J2500 Resto Mod
https://www.fsjnetwork.com/forum/viewt ... 12&t=21395

1974 Bronco “Broncno”
https://classicbroncos.com/forums/threa ... st-3411909

Topic author
Stelzer
Posts: 5
Joined: Sat Jul 10, 2021 10:28 pm

Re: Disk brake questions

Post by Stelzer »

Awesome thank you very much for all the help and information

sansabarJ
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Joined: Sun Jun 16, 2019 1:40 pm

Re: Disk brake questions

Post by sansabarJ »

On your disc conversion , as stated above , get the larger master cylinder at 1 1/4".
Use the very common Corvette unit dual disc reservoir available nearly everywhere
You Cannot use the drum booster number 1 , DO NOT use the tiny aftermarket boosters street hotrod use . TOO LITTLE . EBay etc may sell them and are in use but for space constraints .They will Not provide enough hydraulic action do to small diaphrams . Use the J20 or mid 80s up dual diaphragm Grand Wagoneer boosters . There are universal pushrods available online or Summit has them , offset rod hole or straight , long and short styles needed .I made a late for my 72 J4000 that coveted original firewall holes , pattern used for new booster and then drilled thru once plate was installed . Use a GM type or a Summit Full size disc brake proportioning valve as they are readily available . Do not use the drum brake original valve , in case that's still in the system. I used a J20 GM unit and no issues . Rear calipars ,....ok If large Eldorado 76 77 style new or rebuilt GM parts , the pad to rotor placement is FIRST thing to make sure pads are nearly or are touching the rotors . If a large gap , they will not give any pedal and they will not just travel out by brake pedal pumping , not how they operate . So remove lever spring and lever . Use a 5/8" open end wrench ( if I recall ) may be other , rotate the shaft so it rotates out the piston, just a flat on the Hex of turning usually is good .
The shaft will move out towards you possibly just a bit as you turn or it will move in and you do not want that . Your actuating the screw that moves piston out towards rotor to get your brake pad out against the rotor as much as is possible . You MAY find the lever is NOW unableto be reinstalled back On. You then have to rotate shaft back a FLAT only , to get Lever on the Hex. Take time to SEE what your trying to accomplish in this procedure and watch that the shaft DOES NOT go to far IN the caliper body . Hex cannot go flush to caliper body . Just a precaution , the pad STILL may have a little gap once lever and Springer back in place .usually it's one Hex flat of movement in the adjustment you'll get , not 2 flats of enough movement . So this is crucial to get any rear caliper pedal to work correctly . Once caliper and pads are bolted back in place , you should then have full contact OR near full contact to disc .If a little gap it will be fine , operating the park brake lever and a decent pedal and actuation of brake pedal will provide the pad movement to get the right contact as required .This is critical on the original application let alone a conversion . If your using parking brake calipers you must apply parking brake regular use to keep pads adjusted to the rotor as just pushing on brake pedal does provide this piston movement . Further ALL 4 calipers must be bled in the position of the original app,ication these calipers come from . As stated above take them off the calipar bracket to perform bleeding . Use a wood blockof proper thickness to simulate the rotor . Rears with out park brake you bleed those of course the same as per the fronts . Usual location of front bleeders is about the 2 o'clock position , rears are abut the 10 to 10:30 ish... oclock position for the bleeder screw .
Keep master full at all times during bleeding to prevent an empty reservoir. You CAN probably as a master cylinder option , use a late GM TAHOE master cylinder and proportioning valve for disc brakes front and rear possibly if you want .Late 88 89 90 91 grandwagoneers may also be an earlier year but , used the aluminum GM master cylinders so possible match up of those brake lines and hardware is possible . So do t skimp on required parts and try to NOT mismatch any parts lime a FORD truck booster to GM master cylinder as it causes headaches .


SansabarJ
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