Re: Master Cyl - to bleed or not to bleed after replacing Master-vac

[Rob1972]

As part of replacing the failed brake booster on my 72, I chose to suction most of the fluid from, and remove the master cylinder from my brake lines. Didn't want to risk bending brake tubing.  After the new booster is in place, is it necessary to bench bleed the master, or can I just install it and bleed the lines as normal?

I'm only asking because if I should bench bleed, I need to secure some plugs to block the line connections.

 

Thanks

[ABCar]

Rob, I am kind of in the same boat.  I changed out the master but am now looking at the booster with suspicion...there was seepage between the master and the booster and a lot of corrosion on the booster.

Did you have warning that your booster was going or was it sudden failure?

[Rob1972]

No warning...failed during high rpm downshift.  Rising idle when applying brake pedal.

[Zed Head]

With a bench bleed you can do both front and back at the same time and use hoses run back up in to the reservoirs instead of plugs.  You're not trying to build any pressure just to move the piston through its stroke so that it pushes all air out the bleed ports.  When you're doing it you want to pay attention to where the outlets are and orient them upward so the air bubbles will be there.

Not positive but I think that I "bench" bled mine while it was connected to the hydraulic lines in the car but not bolted tightly to the booster.  So that I could put som etilt on it.  You can also tilt the car to get the parts at the high point.  It's all about the air bubbles.  Be the bubble.

[Rob1972]

ABCar, if you have fluid at the brake booster, it seems to me that can only mean one thing: brake fluid is getting past a seal in the piston of your master cyl.  Brake fluid being very corrosive wouldn't take long to corrode the paint on the booster.  

There is a three step diagnostic to check the brake booster and its associated check valve.  I found it elsewhere using Google, but decided to replace my 44 year old unit anyway without doing the tests.

[Rob1972]

12 minutes ago, Zed Head said:

With a bench bleed you can do both front and back at the same time and use hoses run back up in to the reservoirs instead of plugs.  You're not trying to build any pressure just to move the piston through its stroke so that it pushes all air out the bleed ports.  When you're doing it you want to pay attention to where the outlets are and orient them upward so the air bubbles will be there.

Not positive but I think that I "bench" bled mine while it was connected to the hydraulic lines in the car but not bolted tightly to the booster.  So that I could put som etilt on it.  You can also tilt the car to get the parts at the high point.  It's all about the air bubbles.  Be the bubble.

Thanks Zed Head, I think you are saying that I must bench bleed?  As far as the procedure, the tube runs from the side bleed ports up and into the reservoir - but don't I need to plug the output ports underneath that connect to the hard lines?

[Travel'n Man]

I do a reverse bleed and push fluid into the master to push the air up - works like a charm.

 

[Rob1972]

Thanks Travel'n Man, yeah I've heard of that too. Do you use a Mighty Vac (or similar) to pump the fluid uphill?

To close out this post, I followed Zed Head's advice and put the master on the car and connected the hydraulic lines. I attached tubes to the MC side bleed ports and ran the tubing into the reservoirs.  I jacked up the driver's side of the car to get the bubbles to the bleed side of the MC. My lovely wife pumped the pedal three times and I was good to move on to bleeding the corners.

I'm using a new Motive Power Bleeder for the brakes, I'm hot and cold on it.  I may write up a separate thread on that tomorrow with pics. 

ABCar, did you get up and running?

 

 

[heyitsrama]

I almost finished up my front brakes today, just waiting on some retaining pin hardware. 

The bleeding was interesting, after changing the master cylinder, and removing the calipers to change the seals, it must have took 1.5 min of pumping to get the fluid to start coming out of the bleeder.

I have access to a hand bleeder, ill try removing some more tomorrow after the air has "settled"

 

Kinda scared of the booster failing on me, what happens when it fails? does the depress of the petal get harder? Mine also has that rust between the booster and the master, but I think thats from the old master failing. perhaps I should inspect the booster tomorrow. They seem expensive, $250 to replace? No diaphragm repair?

[siteunseen]

I changed mine out on a '72, it was a "hard' pedal and the PRMs would rise when I depressed the brake pedal.  Surprisingly tough for me and I'm glad I decided not to go with the bigger booster from the 280s, drilling new mounting holes through the firewall.  I loosened the MC from the booster and tied it in place with some wire coming down from my ceiling, pulled the old on out and finagled the new one into place.  Then the PITA part was getting those 4 nuts on the threads above the pedals.  It was tough, for me anyway.  $100 for a remanufactured one that I needed to paint black, it was raw metal when I received it.  If you can get a new booster, the lines flushed of air with new fluid for $250 that's not too expensive after doing the job myself.  I would gladly pay that if I need another one. 

[Rob1972]

Classic symptom is for idle to rise when depressing the brake pedal, as the vacuum seal has been broken allowing fresh air into the intake.

 Replace the check valve also, $40.  I bought my booster from MSA $150 plus a hefty core charge of $300. It looks exactly like my original, down to the Bendix sticker.  Be sure to specify your exact mfg date to get the correct diameter pushrod for your clevis fork.  Then get ready to snake your arms up into the hardest to reach places!

[Travel'n Man]

http://www.ultrarev.com/CARDONE-10-5000MCB-Service-Plus-Brake-Master-Cylinder-Bleeder-Tool.html?imt=1&gclid=Cj0KEQiA-_HDBRD2lomhoufc1JkBEiQA0TVMmjVkg3rz9q8FFQRT5G99HBduxd-EMHWiQWDtum1UwDgaAvpZ8P8HAQ&utm_campaign=Shopping&utm_source=Google&utm_medium=PPC&utm_content=HighPriorityCampaign


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