How challenging is replacing brake master cylinder?

[Rotten 1]

hey folks - finally starting on a few projects on a 1974 Chevy C10 - first up is brakes. The guy I bought the truck from a couple years ago said he "did a brake job but they might need to be bled". I have not done that as brakes were ok, but the last couple of months the brake pedal is soft and mushy, and does not get any firmer after a few pedal pumps. Power brakes, discs in front and drum in back.

My trouble shooting (with a lot of Chat GPT) leads me to thinking that I need to replace the master cylinder.

I'm generally pretty handy but it's been a lot of years since I really got into car repairs, so first question is... how hard is replacing the master cylinder. from a few YouTube videos it looks like the main steps are:
Buy new Master Cylinder
Pull the old one
Bench bleed the new one
install the new one
bleed the brake lines

then go about my merry way.

What am I missing and any tips?

Thanks guys

 

[fast 99]

When bench bleeding do not bottom the piston. Only push it in about 1/2 way. It is possible to cut the lip seal if bottomed out. Won't be as messy if the lid is installed before bolting it on.

On older vehicles I usually check that the bleeders can be opened and are not clogged up before removing the master. If plugged take bleeder off, use a small drill bit dirt can be removed.

 

[75gmck25]

Also make sure you have flare nut wrenches for the brake lines where they attach to the booster. They are going to be tight, and a regular open end wrench won’t have enough bite to get them off without rounding the edges. If you try using vice grips or pliers you may distort the nut or fitting enough that it won’t seal again.

As already mentioned, don’t push the piston down too far when bench bleeding, or after you get it installed and are using the brake pedal to push out the air. Have your assistant use short strokes until you get good pressure. When you push the pedal down too far it often screws up the seal on the piston.

You may have to play around with the proportioning valve to get it to apply pressure to both front and rear. It’s designed to pop to one side and apply pressure to only one half of the system when there is a brake line failure. Bleeding brakes sometimes looks like a failure and it locks to one side.

 

[ali_c20]

Spray some penetrant like WD40 on the lines and fittings. The lines sometimes seize to the fittings and can be twisted off.

 

[Goldie Driver]

Above is all good info. Question, though - have you tried gravity bleeding the brakes ? One man job and you may find the PO was correct that it needed a bleeding session.

 

[ali_c20]

Gravity bleeding is my favorite bleeding method. Watch the brake fluid drip and have a beer.

 

[Rotten 1]

Above is all good info. Question, though - have you tried gravity bleeding the brakes ? One man job and you may find the PO was correct that it needed a bleeding session.

I'll give that a try once this winter event gets done with us as the truck is currently outside. Tell me more about a gravity bleed or do I just go to Youtube?

 

[Ricko1966]

I wouldn't condemn the master cylinder so fast. And I don't have $.10 confidence in the things on chat gtp . Make sure the rear brakes are adjusted, jack up a front and rear wheel have someone step on the brakes,see if front and rear work. If you have some brakes but not good brakes,I don't suspect the master first .Unless the brake light is on

 

[Radiohead]

If you're going with gravity, have the master bench bled completely first.

And ffs, don't run the reservoir dry. Still should finish bleeding the old fashioned way, with a helper pumping the pedal (not all the way to the end of its stroke) and you running the bleed screw, calling out up and down as needed.

Do RR 1st, LR 2nd, RF 3rd and LF last with the rear brakes properly adjusted prior and parking brake off. Don't want its switch on and you thinking it is having a proportion valve hiccup. Keep the reservoir full. Easy peasy man!

 

[Rusty Nail]

Also make sure you have flare nut wrenches for the brake lines where they attach to the booster. They are going to be tight, and a regular open end wrench won’t have enough bite to get them off without rounding the edges. If you try using vice grips or pliers you may distort the nut or fitting enough that it won’t seal again.

As already mentioned, don’t push the piston down too far when bench bleeding, or after you get it installed and are using the brake pedal to push out the air. Have your assistant use short strokes until you get good pressure. When you push the pedal down too far it often screws up the seal on the piston.

You may have to play around with the proportioning valve to get it to apply pressure to both front and rear. It’s designed to pop to one side and apply pressure to only one half of the system when there is a brake line failure. Bleeding brakes sometimes looks like a failure and it locks to one side.

and NOBODY wants their nuts distorted!

 

[Goldie Driver]

I'll give that a try once this winter event gets done with us as the truck is currently outside. Tell me more about a gravity bleed or do I just go to Youtube?

Simple enough. Take the lid off the master cylinder and make sure that the fluid level is full. Put the lid back on loose when full.

Without ever touching the brake pedal, start with the right rear bleeder and let it run until clear fluid (no bubbles) appears.
Tighten the bleeder, check the fluid level and add as needed, then do the left rear.

Follow the same procedure for the right front and left front and reinstall the cap tightly on the master cylinder.

Optional : Follow @ali_c20 's lead and enjoy a brew while waiting and watching fluid for air.

 

[Oldbear42]

Depending on the year and model, you may have a proportioning valve (as mentioned before). Mine is super convenient to get to - just remove your inner fenders and fan shroud... The little lockout tool makes bleeding easier by holding both circuits open. My proportioning valve was so gummed up that I replaced it along with the master cylinder. Took us longer to bleed the brakes than to change the master cylinder, but most of my issues stemmed from the stuck valve. And the rear wheel brake cylinders both leaked so the truck went into "save the front brakes" mode.

 

[Sad Sack]

FWIIW: On the bleeding part, run your bleeder hose with a good seal on the zerk, up a good 6" (think upside down U) or more then back down into a catch jar. Less chance of air getting sucked back in.

 

[Craig Nedrow]

Gravity bleeding is my favorite bleeding method. Watch the brake fluid drip and have a beer.

I wanna have a beer with this guy, I bet they have great beer there!!!!!

 

[ali_c20]

I wanna have a beer with this guy, I bet they have great beer there!!!!!

Yes, we have lots of good beer over here.