I have a ‘75 that had been sitting for a few years. Drive it home noticed right right caliper was leaking so replaced it. Drove it around for a bit no problems. Last week the pedal started getting mushy and the Red BRAKE light started coming on. After talking to a few people seemed like it may be a Master Cylinder Issue. Replaced the Master Cylinder today (did a bench bleed before installing) and then proceeded to bleed each of the brakes 1 by 1. Finally started her up and it’s worse than before.

not sure where to go from here but here’s a few things on my mind:

1. The other 3 calipers are original to the car, they don’t show any signs of leaking but maybe I should change them

2. the brake booster is original, however the car isn’t running odd so it leads me to think it’s fine.

any help or advice is well appreciated

 

did you compare the plunger of m/c s?
different ones take diff length rods.

 

I did not, but the part # is the same so I cannot see that being the cause. Regardless it never solved my original issue.

 

p/n mean squat
can you get your old m/c

 

If you don't see any signs of fluid leaking at the calipers, you don't have any cause to replace them. The only failure they can have is degraded seal to bore contact which is indicated by leaking. When you changed out the master, did you find any fluid where it mounts to the firewall/booster?

Have you changed out the rubber lines to the calipers at all 4 wheels? You could have one that is ballooning under pressure and not know it with a non-pressurized visual inspection.

 

still have it, yes

 

I did not find any fluid, however between the booster and the m/c the paint on the brake booster was coming off which may lead me to think it was leaking at some point? And I have not changed the lines except for the right rear which was replaced along with the caliper

 

ok, good.
you can change plunger, just change to new seals from new m/c If
you don't find other cause

 

"maybe I should change them (the other 3 calipers)?"

when i got my '79 in 2013 it had a leaking caliper. i replaced all 4 calipers/pads from NAPA and no probs since, calipers were $90 ea. i'd replace the rubber lines even if they appear fine, moving up to braided lines if funds permit. my car has a specific procedure when bleeding the rears, not sure about yours.

 

Whatever you do don't buy calipers from Autozone, they are junk.

 

mine were ac delco, $142ea now. looks like autozone carries duralast, never heard of them.

 

Master cylinders can have a deep or shallow hole at the back, compare the new to old.
You need to set the 'gap' (1/32 to 1/16") from the power booster rod to the back of the M/C.
Here's a tool to do that:


Confirm the new M/C creates pressure by removing one hard line and plugging the hole, then reverse the plug and line.
Your brake pedal will be rock hard on both tests if the M/C is good.

Rear calipers have two bleeders, did you bleed the inner side first?

How old are the pads?

 

The best way to bleed these cars is with a pressure bleeder. To bleed them you have to go from the furthest to the closest caliper including the rear 2 bleeders for each caliper. Duralast is the autozone Brand, now refurbished in mexico and they sandblast the whole thing which ruins the cylinder walls. Best bet is Lonestar, CSSBINC (which I like) or WillWood. If you are buying a new master , ensure its for your type, (manual or power brakes) and that it is for disc/disc brake systems. YOu have to match the plunger depth either on both types. Some companies have disc/drum options which you do not want and werent an option on C3s. When I replaced mine I went with a master booster combo because some companies dont have an adjustable plunger for the booster and you have to have or make the correct size piston adapter

 

No one asked, so I will.
What was your method of bleeding procedure?

Pressure
Vacuum
Gravity
Pedal (worst)
Paying someone.

 

For reference I did the brake method with someone sitting in the car.

also this was happening prior to me changing the master cylinder/ bleeding the brakes so I think I’m chasing down something else at this point.

if not then how would I get air in the system prior when they have been working well for a few months?

 

brake issues are one of, if not, the most troublesome issues on c3s.

when right, best feeling, solid stopping, and a major aspect making a c3
what it is.
good luck

 

You may have a 'run-out' problem. Having good brakes and then as you drive the calipers draw air in, did you rebleed and see air in any of the calipers?

How to check the run out:

 

Oh, there's 101 places air can enter the system. I doubt its a "run-out" tolerance issue, because the car has not left the building.

I really dislike the pedal bleeding system. We were all taught how great it is and it does work, has worked for decades. I have done several from "55 Chevys to daily driver pickups. But those other brake pedals have a long throw. C3s do not.
There's where things go wrong. By the time you tell your helper "push" the pedal is already to the floor, then lifts before you can close the bleeder, air enters.

The other issue with pedal bleed is you have a 90% chance you will trip the Proportioning Valve. Its doing its job. It senses a leak. Your bleeder is open, pedal is being pushed, WARNING light is activated.


Before running out and buying more parts, test the MC on the car. Remove the brake lines, one at a time and install a short section of steel brake line, one end pinched off. The front port on the MC has a different diameter and threads per inch than the rear port. You can fabricate "test lines" with help from NAPA and your port hole measurements.
Do Not Use a Bolt to plug the MC port! There is a fabricated / machined flare inside. Once damaged by a common bolt, throw away your new master.

With either port blocked, you should be able to stand on the brake pedal, forever. No movement. Now you know the MC is solid, because you isolated it from the rest of the system.
Re-bleed everything downstream. That M.C. test introduced a bubble.

Boosters seldom have anything to do with a "pedal to the floor". Two separate brake components that work together.
One is strictly an assist and operates by air (vacuum)
The other works by hydraulic (Dot3)

I understand not everyone wants to invest in a Motive Pressure unit. They are pricey, you have to modify them to work on a C3. However, they are a good investment.
Second, the less expensive would be a Mity Vac hand pump. They will really get the DOT3 moving. But sometimes there is a vac leak at the bleeder screw.

Cheapest of all is gravity. Very time consuming but is very successful. Get two plastic bottles, four clear hoses and let the rears drip for hours. Then do the front.
Pull up a chair next to the master with a cold one and a bottle of DOT3. If the front is on ramps, its a little faster.

 

The rubber lines are relatively inexpensive and should be replaced if you're unsure of their age. Once that's done, rebleed and see how it feels.

 

deep hole vs shallow hole