Problem with Brakes

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Problem with Brakes

Postby fas141 » Sat Sep 17, 2022 10:09 am

77 Monza Mirage - I just did the S10 front disc brake conversion. I am using the original proportion valve and bought a new 76-80 Monza master cylinder. No problem getting fluid to all four corners but pedal goes to the floor. Maybe 10% pressure at the bottom and will grab but definitely not right. I thought maybe the new master cylinder might be bad so I bought another but same problems. I thought the conversion said 76-80 Master would work? but not 100% sure that is my problem either? any input appreciated. Thanks.
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Re: Problem with Brakes

Postby cjbiagi » Sat Sep 17, 2022 8:39 pm

Are you bench bleeding the MC before installing it? After bench bleeding you can install two plugs in the outlets of the MC to test it, the pedal should be hard and not go down to the floor. If it does the MCis either bad or has air in it.
Clyde.........75 Monza 2+2
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Re: Problem with Brakes

Postby monzaaddict » Sat Sep 17, 2022 9:03 pm

if you used the single piston s10 calipers the 76-80 master should work. Dual piston needs larger master cylinder. manual brake master attaches to pedal at top hole. power brake booster attaches at lower hole.
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Re: Problem with Brakes

Postby fas141 » Sun Sep 18, 2022 4:51 am

Yes, I did bench bleed it and could only push the plunger about 1/4"..

I used the single caliper conversion. Car has PB but I did not change anything there..

I am at a total loss.. I get a steady stream of fluid when I open a brake cylinder (while someone is pushing the pedal down) but its only holding pressure at about 10% of the pedal distance at the bottom.
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Re: Problem with Brakes

Postby monzaaddict » Sun Sep 18, 2022 9:52 am

are the bleeders at the tops of the calipers? Arre the hoses so long that they extend above the calipers trapping air in the loop? Are the rear brake shoes adjusted properly?
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Re: Problem with Brakes

Postby monzaaddict » Sun Sep 18, 2022 5:37 pm

The plunger should move more than 1/4”. When you bench bled the master did you have the adapters with the hoses that loop back up to reservoirs?
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Re: Problem with Brakes

Postby cosvega76 » Mon Sep 19, 2022 8:25 am

If you are bench bleeding the master with the ports plugged, you shouldn't be able to push the master cylinder piston in more than 1/8" if it's fully bled. If it's 1/4", you still have air in there.


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Re: Problem with Brakes

Postby Lane Meyer » Thu Jan 05, 2023 12:47 pm

Did you resolve your issue? What worked?

I have a similar issue in a '74 Vega wagon w/stock power brakes that I recently purchased. I haven't looked too deep into it yet. So far:

Bench bled the MC (in the car) and bled all lines
Calipers are on correctly (bleeders up)
Rear drums adjusted correctly
Good vacuum for power booster, good check valve
Power booster tests* OK

* With the car off I pump the pedal several times and hold. When I start the car, the pedal drops.

My next step will be to remove the MC so that I can bench bleed and test w/plugs. That'll give me the opportunity to inspect the MC and booster as well.

Unfortunately I don't know the history of the MC that is installed. It's certainly possible that it is incorrect. It doesn't look like a direct replacement MC is readily available.

Thanks!
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Re: Problem with Brakes

Postby monzaaddict » Tue Jan 10, 2023 1:58 am

https://www.rockauto.com/en/catalog/che ... inder,1836
RAYBESTOS MC36441
check that the clips on the rear parking brake adjuster struts are not broken. bleed the master level in a vice not at an angle on power brake booster.
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Re: Problem with Brakes

Postby Lane Meyer » Tue Jan 10, 2023 9:25 am

Appreciate the link. I searched Rock Auto a few weeks back but all of the power brake units were out of stock. I may have been barking up the wrong tree - I don't recall what year I was searching for.

I did jack the rear end up to get the MC as close to level as possible when I bled it but it was still tilted to the rear. That'll be my next step.

"check that the clips on the rear parking brake adjuster struts are not broken" - I'm not following. Is the parking brake not a traditional single/double cable with an equalizer setup? The parking brake works great so I haven't given it a look at all.

Thanks!
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Re: Problem with Brakes

Postby cosvega76 » Wed Jan 11, 2023 9:09 am

I think what he is referring to is the unusual drum brake adjusting system on the '71-'75 Vegas. Rather than a traditional starwheel adjuster, they used a strut between the shoes that had a friction tab/insert for the automatic adjuster. It was a somewhat fragile design, and was replaced by the starwheel adjuster design in '76.


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Re: Problem with Brakes

Postby Lane Meyer » Wed Jan 11, 2023 10:22 am

cosvega76 wrote:I think what he is referring to is the unusual drum brake adjusting system on the '71-'75 Vegas. Rather than a traditional starwheel adjuster, they used a strut between the shoes that had a friction tab/insert for the automatic adjuster. It was a somewhat fragile design, and was replaced by the starwheel adjuster design in '76.


Chuck


Thanks, I appreciate that info. I haven't done rear drum brakes on a Vega since 1986... can't say that I recall. :)
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