Sorry about the long post, just trying to give as much detail to whoever reads this.

1970 Corvette coupe, 350/300HP, manual brakes, power steering, 4 speed manual trans.
I replaced front and rear brake pads along with front and rear brake hoses, master cylinder, front and rear crossover lines, and the small steel lines that run along the trailing arms to the rear calipers. I've tried every method I can find here in the forum pages but just can't get even close to a hard brake pedal. More like a wet sponge.
After replacing the lines and hoses I installed the master cylinder after bench bleeding it I bled it while level, while tilted forward and backward, all the while tapping it with a wooden mallet until bubbles stopped coming out of the 2 tiny bleed holes in the bottom of the m/cyl. (Did this for at least 30 minutes until bubbles stopped coming from the bleed holes, then another 10 minutes to insure no more bubbles. Must have tapped it 500 times.) I installed the master cylinder and proceeded to bleed the rest of the system.
I tried:
-Pedal bleeding twice
-Gravity bleeding twice
-vacuum bleeding twice
-bought a Motive pressure bleeder and pressure bled the system
-removed the new master cylinder and bench bled it again. It didn't appear to have any air it beyond what got into it during removal.
-did another pressure bleed after reinstalling the master cylinder. Got several bubbles out, probably just the air that got into the system when the m/cyl was removed.
-did another pressure bleed, just in case.
-ran out of ideas, had a beer and abandoned the job for several weeks.

After the initial pedal bleeding there were NO bubbles at all coming from any of the bleeder screws. Same with all the rest of the methods. I watched for this very closely. There is very little difference in the pedal from before the initial bleed to where it's at now. There are no leaks anywhere in the system. I pressurized the brake system to 12 psi with the motive bleeder and left it overnight. There were and still aren't any leaks.
What am I missing/not doing/doing wrong here? Maybe should be looking elsewhere? Brake pressure switch? It seems to working alright. Out of ideas, will appreciate your input!

 

Does your master cylinder have one of these bolted underneath it?

There is a symmetrical valve/piston w/o-rings inside the block. If the piston shifts to one end of the block because of uneven pressure (leaking front or rear brake sub-system OR because of replacing brake components) it will prevent the brake system from being bled completely. It may or may not be the problem.
Let us know if the car has one installed.

 

Everything looks good. I just question if the MC was damaged during the bench bleeding.

When you insert the screwdriver to pump the MC piston, if you go more than 1 3/8" you could damage the seals. With the MC in the vise, it can falsely seem like its building pressure. But when its installed in the vehicle, the brake pedal exerts far more leverage & pressure on the MC seals.
You will never get a firm pedal if fluid is slipping past the MC seals.
And bleeding the system will be difficult w/o the MC at 100%.

 

man, that’s a tough one. I know you said no leaks but I keep thinking that’s what it is. The proportioning valve doorgunner mentions is worth checking into or replacing. Do you think it’s possible you got a bad mc? Mine is a 70 and I have that proportioning valve. Just replaced my mc and bled after not having any issues with my last mc. I had to do this because the pedal lost all pressure during initial start up of the new build. I still don’t know how that happened but it did. I’m towards the end of my build and never had issues with the brakes prior but bled and even troubleshot the booster. As it stands right now (haven’t been able to drive it yet) it seems changing the mc has firmed up my pedal.

 

Definitely didn't damage the MC. Had a small vice grip pliers clamped at the 1 3/8" mark onto the Phillips screwdriver I used to bleed it, so that I wouldn't drive the piston beyond that depth.

 

One other thing: Make sure all the brake hose threaded connectors have NEW copper washers installed to seal against the caliper bodies....bad/no copper washers will allow air to be sucked into the calipers...mine did not leak BUT I continuously had air in two of the calipers = spongy brake/low brake pedal.
After 6 months they decided to leak..."Blind squirrel finds problem".

 

DG, thanks for commenting here. My car does have one, it's in a slightly different location than yours. (see pic.) If the piston in the junction block is tripped because of unequal pressure, wouldn't that cause the brake warning light to come on? It's not on when I turn the ignition key to ON position. I know the light works because it illuminates when I set the parking brake.

 

Yep, all new copper washers installed where required. Good point though.

 

I'm thinking maybe a bad master cylinder at this point, I'm not sure what else it could be. Maybe pull the lines off the master cylinder and plug the ports and see if you have a stiff pedal, the ports are 1/2x20 and 9/16x18. If it is stiff when plugged, it must be something downstream.

 

Nowhere in your posts have you mentioned if the brakes work or not on a test drive.

 

That's true. The car is still up on jack stands. I haven't test driven it because the pedal is so soft that I don't think it would hold the car even on my very gently sloped driveway.

 

The brake pedal feels mushy but you never said it went to the floor. Have a helper push on the brake pedal and see if the hubs can be rotated using a screwdriver or bar wedged between two wheel studs. If they can't be rotated, put wheels on, lower car to floor and put car in gear to see if it holds. If it does, well, test drive it.

 

So I follow this procedure to maximize braking for my C3 corvette. ( I use my car for road course track days )

#1 bleed the master cylinder while on the car and connected to the brake lines.

here is a post of what I do.

Get the last bit of air out from the brake master cylinder. - CorvetteForum - Chevrolet Corvette Forum Discussion

I repeat this process every day until I get no air bubbles. I have once needed 3 days to get it perfect. Meaning I left the car with the back end raised for 3 days.

Also try leaving the car raised for 60 minutes and repeat.

Once that is completed
#2 at each wheel clamp the pads to force the caliper pistons in as far as they can go. Then bleed the caliper by applying pressure at the MC.

Here is a picture of what it looks like for me. I use ~ 20 PSI at the MC

 

Thanks for all your comments.
I finally got back to this for a short time yesterday. The first thing I did was to check what kind of force, if any, is being applied to the calipers when the spongy brake is depressed. I goes near, but not all the way down to the floor. With a helper holding the pedal down firmly I tried to turn the rotors with a 3 foot pry bar. They wouldn't move no matter how much force I applied. With the pedal released they turn easily by hand. So there is some clamping force being applied to the rotors. The next thing I did was to isolate the master cylinder as was mentioned above. With the two brake lines removed from the MC and the ports plugged, I applied the brake pedal and it was like stepping on a rock. After the initial pedal free play the pedal was completely solid. So now I know that the MC is working as it should. I also checked the brake pressure differential switch in the front distribution block. The brake warning lamp in the dash doesn't illuminate when the ignition is turned on so to check its functionality I grounded the pressure switch lead wire to the engine block and the warning light came on. Connect it back to the distribution block, the warning light goes off. This tells me that the pressure differential switch is centered and working normally. After all this, I re-connected the front brake lines and bled the system again with Motive bleeder to purge the air that got in when I removed the lines from the MC. I saw several large bubbles and a few smaller ones emerge from both the rear and front systems. That was no doubt the air from when the lines were removed. I checked the pedal again, still mushy.
SO: -I know that the MC is good and has no air in it
-I know that the brake pressure differential switch is centered and not tripped
-I know that some braking force is being applied to both front and rear calipers
-the front and rear calipers are both fully retracted so they shouldn't hold much air (new Powerstop brake pads installed and they're thick enough to fully retract the calipers)
It seems then that there has to be air trapped somewhere in either the brake lines or the calipers. Repeated pressure bleeding has failed to remove it. What should my next move be?

 

I can’t explain why but, a few weeks ago I pressure tested the mc just like you did and got the same results you did. I ended up reading somewhere that it could still be the mc. I found a really good deal on one and said what the heck. Easy enough to change so why not. Did it and my brakes worked after bleeding. I bench bled the new mc and gravity bled the calipers. Just thought I’d mention that to you.

 

Only thing I can think of at this point is the four rubber lines. They can be internally damaged and allow pedal pressure to bulge the hose and get a mushy pedal, unseen from drivers seat. Only if you are watching for this, would you ever know of its existence. So you need a helper on the pedal while you do a visual.

These C3 brakes are frustrating, no doubt. Never seen a brake system that causes so much negative feedback. I have been doing brake jobs since drums / shoes on all four corners and a single master cylinder. Those were a lot easier.

Keep at it. One day everything will fall into place.