70 series Brake master and booster combo question

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Hi all,

A while ago, my stock MC started to fail on my HZJ77, I had an MC from an 80 series handy so I used it to replace the stock one. Bolt pattern matched, both vehicles have front and rear disc brakes so I figured it should work the same. Been running this for a while and the brakes work fine, but I did start noticing some sponginess once fully depressed. Best way I can describe is it gets firm upon initial depression, but then there is a slow give where the pedal continues to travel. When this happens the brake light on the dash illuminates intermittently. I thought that light only comes on with the ebrake and if the fluid in the MC gets too low? Definitely neither is happening when it flickers on/off. There is a wire that comes off the vacuum reservoir, is that light monitoring vacuum as well?

I've bled the system until blue in the face, lines are all new, calipers are new.

So my question is could it be the mismatched MC? Are Boosters and MC designed specific to work with one another? Or is it vacuum related?

Thanks all,
 
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I don’t think mismatching should be a problem. Mine came with I think a booster from a FJ cruiser. When I rebuilt my Ute I got a factory booster. I have a vacuum leak I haven’t chased down yet since the brakes and clutch still work fine then the engine is running, but definitely loose vacuum pretty fast then engine is off.
 
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Hi all,

A while ago, my stock MC started to fail on my HZJ77, I had an MC from an 80 series handy so I used it to replace the stock one. Bolt pattern matched, both vehicles have front and rear disc brakes so I figured it should work the same. Been running this for a while and the brakes work fine, but I did start noticing some sponginess once fully depressed. Best way I can describe is it gets firm upon initial depression, but then there is a slow give where the pedal continues to travel. When this happens the brake light on the dash illuminates intermittently. I thought that light only comes on with the ebrake and if the fluid in the MC gets too low? Definitely neither is happening when it flickers on/off. There is a wire that comes off the vacuum reservoir, is that light monitoring vacuum as well?

I've bled the system until blue in the face, lines are all new, calipers are new.

So my question is could it be the mismatched MC? Are Boosters and MC designed specific to work with one another? Or is it vacuum related?

Thanks all,
I did the same thing to my BJ74 with the factory master and it actually gave my brakes a tad more oomph. I had to spend some time adjusting the pedal and making sure the engagement between the master and booster were right. Did you adjust the rod coming out the of the booster to match the length of the old booster? If not that could be your "squish" because the master isn't being engaged as quickly as it would otherwise. The other option is a vacuum leak or air in the brake lines, if you're not losing fluid that's really all that's left.
 

jblueridge

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If you have the LSPV, detach its arm and bleed the rears with the arm all the way up then all the way down. I think there is a return line from the LSPV that can get air in it.
I ended by replacing the LSPV with a new oem unit.
Yes, that brake light comes on for low vacuum too.
 
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Went out today to try of few things after its been sitting a few days. Started the truck and pedal was rock hard like no vacuum was present, and the dash light was on, and clutch was hard as well. Pulled the Vac line off the booster could feel vacuum being drawn by the pump, plugged in a gauge, its pulling about 28" from what i could find online thats normal. With the vac line to the booster plugged, light went off and clutch was working fine.

Hooked up a hand vac pump to the booster to see if it will draw vacuum and hold it on the gauge, nothing. Tried the same thing on another booster and I could get 10" with the hand pump.
So it looks like the booster is leaking by somewhere, probably has been getting worse little by little until it finally failed fully.

This is the part number for a HZJ77,

4461060460​

Only available from partsoque at about $500, anyone know a comparable replacement. These do have to specific so the rod length matches with the pedal. Otherwise looks like I am waiting 2 weeks for a new booster.
 

jblueridge

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I put an early 80 series booster on mine to match the 80 series MC.
This required making a new turbo heatshield and lengthening the pushrod.
I also added an extra vac reservoir since the dual diaphragm seemed to need more vac.
 
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Went out today to try of few things after its been sitting a few days. Started the truck and pedal was rock hard like no vacuum was present, and the dash light was on, and clutch was hard as well. Pulled the Vac line off the booster could feel vacuum being drawn by the pump, plugged in a gauge, its pulling about 28" from what i could find online thats normal. With the vac line to the booster plugged, light went off and clutch was working fine.

Hooked up a hand vac pump to the booster to see if it will draw vacuum and hold it on the gauge, nothing. Tried the same thing on another booster and I could get 10" with the hand pump.
So it looks like the booster is leaking by somewhere, probably has been getting worse little by little until it finally failed fully.

This is the part number for a HZJ77,

4461060460​

Only available from partsoque at about $500, anyone know a comparable replacement. These do have to specific so the rod length matches with the pedal. Otherwise looks like I am waiting 2 weeks for a new booster.
Have you checked into your clutch booster seal? Took me a second to wrap my head around that with mine, had a clutch booster leak that threw me for a loop, made my brake booster weird and kept my shutoff solenoid not work (13B-T). Was your 80 booster new or used?
 
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@jblueridge how did you determine it needed more vacuum. When i first did it, I put the 80 series booster on mine as well, but it didn't work right, I figured the diesel didn't make enough vacuum for it so I put the stock booster back on. How was yours acting before you put the extra reservoir on?
 

jblueridge

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The booster was a used one and I took it to White Post Restorations for a rebuild. After that, it was easy to figure out that a larger reserve of vacuum was needed.
If I pumped the brake pedal, it felt like I was pushing a rock on the 3rd pump: booster was no help.
The bigger booster takes bigger bites of vacuum.
 
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The booster was a used one and I took it to White Post Restorations for a rebuild. After that, it was easy to figure out that a larger reserve of vacuum was needed.
If I pumped the brake pedal, it felt like I was pushing a rock on the 3rd pump: booster was no help.
The bigger booster takes bigger bites of vacuum.
What did you use for bigger canister?
 

hoser

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I would consider going with one of the Dual Diaphragm brake boosters from CityRacer.


I see they now carry a 40/50/60 version... unsure what the differences are.


I also went with a larger vacuum canister.... not completely sure if it was needed though.

 

jblueridge

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What did you use for bigger canister
 

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