All new/rebuild components. Bled, bled and re-bled. Hard Pedal with engine off. VERY soft pedal when the engine is one. The pedal will bottom out if I don't pump it a time or two with every use. I have bled QUARTS through the lines and vacuum and gravity bled and I am wondering now if my master cylinder is too large. I have read instances where people swapped to a smaller one and problems went away but I really thought I had the right components here.

Any ideas or theories?

I'm assuming the rear drums are up & pulling on the ebrake will ans that Q in seconds. When they pump up do they have to be pumped up again shortly thereafter? I'm assuming you bled the MC first before starting on the corners? Is the rounded pushrod nub a hair away from the bottom of the MC piston recess "the thimble"? You might grab a pair of brass inverted flare male brass fittings at the Edelman cabinet at your parts house & cap the MC ports (they're cheap & you will reuse em in the future). Then with it capped/idling if the pedal is rock hard with virtually no travel then the MC is good & the MC is bled out then uncap the drum half & hook it up & bleed it out till the pedal is hard then connect/bleed the front discs.

I've had brand new master cylinders bad right out of the box and even a quality resleeved unit bad once. Don't trust the it's new so it must be good method of diagnosis. As stated above cap off the lines to isolate problem front or rear. If that doesn't yield a problem check service manual for spec on bore diameter of master cylinder. Then check what your replacement has. A 1-1/8 master cyl bore will yield a high and hard pedal even with a power booster. Switching to a 1.00" or even smaller piston (like some mid'70's A-body) will yield the feel of more power assist however the pedal will be noticeably softer and lower. A good tool to have is a brake pressure gauge. It screws in where the bleeder valve goes and you can start figuring out where your problem lies.

Just a thought.The calipers are installed with the bleeder up? Seen it before.

Yup Bleeders up. I am heading to the part store this morning to get a couple of plugs. I hope it is the MC since that will be a pretty easy fix.

Went to two Auto Parts store and sorted through flare fittings and had no luck finding anything to plug those holes. It would be easier to order another MC! It would suck to have to drive 40 miles or special order plugs for this test. I would almost rather spend $50 and get a new MC locally.

Looking now to see if I can figure out the threads to order something.

They are straight threads so I would measure the OD & I would assume that they ain't fine threads. What I would do is grab a bolt out of your junk that fits. As you know it is an inverted flare fitting if that might help on finding the correct thread type so you can let your fingers do the walking & order it by ph or online. Holler how things turn out

What's the masters application?....did you bench bleed the master before connecting it to the system?, calipers,Stock, aftermarket? pin style, floaters?, proportioning valve?, distribution block?, is this a recent drum/drum to disc/drum swap?, ...some background on your set up rather than 50 questions trying to determine what you have, helps us, help you



Mike

Thanks all. I found that bolts worked to block the MC. I used a 9/16 - 18 thread and 1/2 20 thread. With both ports blocked, the MC was rock hard with and without power assist. I reconnected the fronts, bled them and the pedal got soft. I then reversed it to test the rears. Re-bled and the pedal was soft. At least I now know I have at least one good part.

The front calipers are the pin type and they were purchased from O'Reily's as rebuilt calipers for a 70 Cuda. The bleeder valves are up high. I bought rebuilt wheel cylinders from O'Reily's for the rear and all new springs, shoes, pins, etc. Brand new SS lines were purchased from Inline for front and back and I am using the 2-piece Distribution Block and Hold Off Valve from Inline Tube.

This master is designed to be used with Disc Front and Drum Rears. I bought it from O'Reilys for a 70 Barracuda. I bench bled it before I installed it and once I installed all the components, I ran about 2 quarts through the system and then bled, bled, bled using various methods.

I posted a photo of the master and booster and this photo shows the distribution block and hold-off valve which is as close to stock as I could afford to get.

I assume that the next step will be to block off the distribution valve output and retest.

Does this seem the natural progression?

With the front discs capped is the pedal hard? If not I would keep the front disc half capped & bleed out the rear drums (pull the ebrake will help you with them) till they're good. If yes (pedal hard which means the drums are bled) I'd work on the discs & pumping the pedal fast 3 times then having a helper open the bleeder on the 4th downstroke helps agitate the fluid & get the air out. Perfect verbal timing with the helper. Put a block of wood of the right thickness under the brake pedal to limit its' travel to the normal range & it makes it easier for you to come to a dead stop. On a real tough case you might need to take off the caliper(s) & block the piston solid then tilt it as needed till the air is out. Just cause the bleeder is up does not neccessarily mean where the passage meets the bore that that point is at 12 o'clock. Most times agitating the fluid will get the air out. No need to undo the prop etc line fittings as long as the fittings are tight/no leaks

When I did the plug test, I bled each side as you described and the brakes were still soft. The only thing I did not do it put a block under the pedal.

I am going to block the front and bleed the backs until I can get them stiff or detect a leak. I have plenty of fluid and can bleed, bleed and check and check.

I have the car lifted so I can get under the distribution block and hold valves to ensure there are no leaks.

Thanks again for all your help.

With the car lifted, I had an assistant pump the brakes about 50 times with the engine off (no power) and again with the car running and with a light I saw zero leaks but I did notice the piston moving on the metering (hold off) valve. The piston seemed a little moist but no drips. I wonder if it is possible the metering valve has a leak and it is pulling in air, but if that was true I imagine it would leak fluid as well.

Also, when I first installed the distribution block, I did notice a small fluid leak around the switch assembly where the warning light connects. I tightened the outer nut and the leak disappeared.

I only mention these things because they came to mind. I am stepping out now to plug the fronts and bleed the crap out of the rears.

Just got back from a test drive with only the backs connected and 1/2 quart of fluid bled through the lines. It feels better, but I am still concerned.

The sponginess goes away about halfway down. Then there is a pretty firm feel but since these brakes are new to me, I am not sure if the stiffness is indeed pressure form the wheel cylinders or if the pedal is as far down as it will go. I did not pay much attention to the play when I installed them so I am not sure where "bottom" is.

There is about 1/2" to 3/4" of pedal movement before you can feel the piston start to engage which seems pretty good to me.

Also, I saw no bubbles when I was bleeding and so I bled them with the engine running and was able to see and extract some air pockets that I did not get out before. I don't think this is the proper way to bleed but it did seem to help agitate a little more air out.

Tomorrow I am going to take it for another test drive and make an effort to document where "bottom" is on this pedal by getting someone else to open the valve while I measure the distance to the floor during a bleed. Once I am satisfied, I will reconnect the front's and get them bled and repeat the process.

Thanks for helping me along with this guys. I needed some fresh ideas and am still open for suggestions if anyone has them!

You guys are the best.

-Steve

Just to reaffirm your bleeding technique, when doing the rears, your starting from the passenger side correct, then moving over to the drivers rear?...your assistant is fully aware of the proper pumping pedal/hold procedure as you bleed the brakes (as I've gone thru this "ordeal" many times with an assistant, only to find they are the culprit in improperly bleeding a system)...when you bleed the fronts, your starting with the passenger side and finishing with the drivers correct?


On another note, sometimes disc calipers have casting voids that retain air during the bleeding process, only to release it later, causing the system to fail...one remedy, is to remove te caliper from it's mount, use a C clamp and carefully collapse the piston, slowly bottoming it out in it's bore (watch the fluid level in the master when doing this) then SLOWLY and with little pedal pressure bleed the caliper(s) one at a time with the C clamp keeping the caliper piston compressed, this will eliminate any trapped air in a casting void...

Another thing to consider, is the fluid,... are you returning the "pumped" and collected/bleed fluid back into the system?, if so if may have been aerated with micro bubbles, that when under compression in the system cause a "spongy" pedal, bleed fluid should not be returned to the system,


Hope this might help, I know bleeding any hyd. system can sometimes be fraught with headaches


Mike

Another item to consider:did you adjust the clearance gap on the booster rod before installing the master?

Mike

Great comment! No, I adjusted nothing. The booster (shown) does have what looks like a threaded cap on the rod but I am unclear as to how I measure the gap inside the MC. My best guess would be to measure the depth of the piston inside the MC and then measure the length of the rod on the booster and then thread in or out the cap until I reach .20 ?

Thanks for the post. I can see where an oversized gap there could give me too much pedal play and a pedal that sits too low.

Sound right to you?

DAYCLONA,

That was it! The sponginess was the HUGE distance the pedal had to move before the booster rod ever engaged the MC piston. No joke, I had to adjust it out nearly 1/2" to get the .020. I do not know for a fact that I am .020 but I got it as close as I could without touching.

When I said the pedal was on the floor, now I know exactly why.

Now that the booster rod does not need to travel so far, the "bottom" of the brake pedal is easily 1.5 inches higher than it was and now you can ease on the pedal and get braking results.

Excellent, excellent, perfect tip. Tomorrow I will reconnect the fronts, bleed the system again and then I think I am done.

Thanks again to everyone who helped. I will pass this knowledge on to the next person who needs it.

Good night.

-Steve

RapidRobert, you also recommended the push rod adjustment but it did not register in my head when I read it.

Thank you very much for helping me out and I apologize for not picking up on the tip when you brought it up. Somehow my brain skipped over that entire sentence and I should have paid much closer attention.

As usual, the expertise and zeal of this group has proved exceptional!

Thanks again.

Steve, you can use a set of dial calipers to measure the depth of the piston in the master 's bore, by using the slider end that slides out when you slide them open...improper adjustment will cause the front discs to drag, causing heated rotors/warped rotors, fluid boil, glazed pads so adjust for a gap of .020 to.060 max



If your unfamiliar with the "depth gauge" section of dial calipers, here's a pic...

Mike