1969 Fleetwood 75 master cylinder / brakes issue

[JokerLimo]

I recently put my 1969 Fleetwood in the shop for the brakes to be done. I
was told that i needed a new master cylinder and should get a booster
while i'm at it, as the rear brakes could not be bled properly. So i had a
shop rebuild master cylinder and the booster. upon getting them back I am
told they still can't get the rear brakes to bleed or get all the air out.
( same issue as before) so ,the shop seems to think its the master
cylinder not producing enough volume to the rear brakes. I watched them
bench bleed the master cylinder and it did seem the front pumped a lot
more volume than the rear section or port.  I hate to think its the
rebuild job. what could be the problem?
any suggestions or comments greatly accepted.

[Ralph Messina CLC 4937]

The brake lines may corroded or collapsing on the inside. Take a line off and see whether you have flow when applying the brakes.

[m-mman]

Yup, the rubber lines commonly collapse on the inside and can look fine on the outside. 
They are 46 y/o rubber, replace ALL THREE!!

Replacing the rubber lines is a mandatory part of a brake job on any old car. Likely your shop is not used to problems associated with vehicles that are this old.

[Ken Perry]

Sounds like a proportioning valve problem to me. Once they stick to the front or rear they are hard to get to let go and center again.They will shut off back or front lines. Take it off and clean or open up the other side and hit the pedal hard and sometimes they will unstick. This is the kind of problem that would come up Friday eve when you want to go home and all you need to do is bleed the brakes and the people can have there car,ask me how I know that ! Even way back they were dealer only part and not in stock . Maybe you can get a new one somewhere? There is a botton on back of some of them that is to release them,but never seem to work. Hope this helps. Ken Perry     

[Scot Minesinger]

All three posts seem likely, bad steel lines rusting inside, bad rubber line in back, and a possible proportioning valve issue.  The rubber line is where I would start.  The idea that the mechanic suggested a new master cylinder to cure this problem is disturbing.  Although if this replaced original booster and master then it was time anyway.

[JokerLimo]

ok thanks guys.
I believe the mechanic replaced the rubber line at the rear. I think there was only one.
 I'll check into getting a proportioning valve also.
i watched  the mechanic bench bleed the master cylinder and the front volume was way higher than the rear. the rear was barely pumping out compared the the front.

[Ken Perry]

Keep in mind that your car is front disk brakes,way more volume for the front. Ken Perry