Like alot of you, I have been working on my '69 brakes for a long time.
The car was a bubba special when I got it, so I don't always know if parts are origional or replaced. All the major brake parts have been replaced. Some replacement parts were bad out of the box, which always messes up the troubleshooting process.
I recently replaced the master cylinder and the system worked pretty well, not perfect, but ok.
I think the replacement booster is a later model--it has a recess for the mc to fit into, rather than being flat as the origional is suposed to be.
I think the rod to the mc is the correct length.
So, here's the problem. Brakes seem to work ok. After 30-40 miles, the brakes just don't grab. Pedal is real hard, but the the brakes are only working about 40%. I cannot lock up any wheel.
I did change the hose to the booster, because it was labled as a fuel hose and i thought it might be colasping. No change.
When I first installed and bled the new mc. I could see bypass fluid pump in both of the resesovirs when the pedal was pumped. Now I can only see bypass fluid in the front chamber of the mc. Incorrect booster rod length or bad mc?
I also checked the booster vac check valve and it may be leaking a little. I'll change that also, although I don't think this is the main problem.
So, any ideas?

 

The master may have failed based on your description.

 

Hard pedal and not much braking makes it sound like a problem with the booster.

OR

The rod is too long and is keeping the brake pads in contact with the rotor and heating them up thereby making them ineffective.

 

The mc holds pressure when the pedal is pushed, so it is not leaking down. Can it fail internally and not push the piston, if the pedal does move about 1"?

If I apply pedal pressure with the engine off, the pedal will drop a little when I start the engine. This is the booster test I researched. This was my thought also, and why I changed the vac hose to the booster. Any other tests?

I will check the rod length again, but the problem is evident now on the first stop before the brakes could get hot.

Any ideas on if, when stopped and the pedal is pushed, there should be bypass fluid flow in both resevoirs of the mc? I see alot in the front section, but none in the rear section.

 

When you got the booster...if it is like you mentioned being the later design...it all depends on the backside of the master cylinder.

SO..if the master cylinder is CORRECT for the era...and has a hole in it which is quite deep...where the rod form the booster goes into...then the booster should have come with two rods. One short and one long.

IF the master cylinder has very little to no hole in the piston of the master cylinder then you use the short rod.

I have run ingot his where the check valve will not flow vacuum as required....and when I changed it...the brakes worked perfectly.

If you go in and press on the brake pedal and the crank the engine...the pedal should go down some. If it does not...then it is in the booster 99% of the time...with the exclusion of the check valve being OK.

I know what I am doing and know what I am feeling for...so I am NOT endorsing anyone to do this. But knowing that vacuum is required to make the power brakes to work...if you pull down the driveway where it is LEVEL and SAFE to do this. And test the brakes and if the brakes do not work well at all.....then I remove the vacuum going to the booster and plug it and check it again...if there is NO CHANGE in how the brakes work...LOGIC would command that I would have an issue with vacuum...either the booster or the check valve...under the assumption that I know I have good vacuum to start with.

DUB

 

I think I have a newer style (75-78) booster (recessed) and a 69 style mc (flat). Got the booster off ebay a few years ago and if I remember correctly, it listed it as fitting 68-80 or 82 or whatever, that I have since read that they are the later style. The mc does have about a 1 1/2" hole in the piston, and the booster rod (non adjustable) does stick out about the same distance.
But, it all worked ok for 30-40 miles then failed.
I'll get the check valve tomorrow and check it out.
Thanks all

 

I changed the power booster check valve and the brakes are back to normal again.
Never would have figured this to be the problem. It wasn't blocked, I could blow through it 1 way, and it would mostly check/stop in the other direction.
Thanks for the suggestions.

 

That's good to hear....also....it took me 20 minutes to bench bleed my m c before I got good return fluid out of the rear reservoir .

 

AWESOME...Like I wrote..it happened to me and I am glad to read that your brakes are good again.

YES...this check valve is just that...works in one direction...thus allowing you to be able to pull air through it like the engine would and create vacuum in the booster....but if the check valve is really hard to get operate...it effects the performance of the booster. The check valve needs to be very easy for it to open and create a vacuum.

DUB