What causes brake fluid to move from rear to front reservoir?

[72farmer]

I have a 67 C-10 that i just bought that has a brake problem.The pedal is soft and it travels a long way untill you get brakes.It has drums all the way around and a power booster.I removed cover on master cylinder a week ago and the back reservoir was dry.It goes to rear brakes right? I replaced the 2 front hoses going to the front brake wheel cylinders, bled brakes all the around and refilled master cylinder. I took the drums off and inspected brakes and parts and every thing looks new and correct.When i took the cover off of the master cylinder yesterday the back reservoir was dry again and alot of fluid ran out of the front reservoir.It has no visible brake fluid leaks to cause the back reservoir of the master cylinder to be dry.What could be causing this?

[cdowns]

i'd think bad master cylinder if you can find no leaks // would check the whole length of the brake lines real good for rotted out spot where it leaks first tho

[1968SWBBigBlock]

you have a bad seal on the plunger assembly
You can go old school and replace just the seals and rebuild the MC yourself or its time for a new MC from the local autoparts store
Good time to flush the entire system to insure that you get any small debris out of the system
This failure normally occurs from wear and use or foreign objects in the brake fluid

[7T2]

If you need to get a master cylinder, save yourself a lot of hassle and get a new one, not a rebuilt/remanufactured one. The last time I did a master cylinder I went through three or four remanufactured ones from NAPA that all leaked. Luckily they leaked when I was bench-bleeding them so I didn't get into a lot of work installing and removing them. Took the last one back and got a brand new one - worked like a champ. It costs about twice as much, but its worth it to avoid this problem. I've had it happen on two different vehicles.

[mbgmike]

Originally Posted by 7T2

If you need to get a master cylinder, save yourself a lot of hassle and get a new one, not a rebuilt/remanufactured one. The last time I did a master cylinder I went through three or four remanufactured ones from NAPA that all leaked. Luckily they leaked when I was bench-bleeding them so I didn't get into a lot of work installing and removing them. Took the last one back and got a brand new one - worked like a champ. It costs about twice as much, but its worth it to avoid this problem. I've had it happen on two different vehicles.

Good advice. IMO its not worth it either.

[Jacfourteen]

For the price just go get a master cyl, reman cost me $17, so new is probably about $35.

[superfly66]

I know this is an old thread but, I'm having these issues as well. I put all new brake parts on front to back. Disc on front, all new lines, new MC, new Prop. valve, new rear drums parts, new booster. I read this thread a few weeks ago and replaced my already new master cylinder with a new one. I'm having the same issues. Back reservoir fluid moving to the front reservoir. 2 new MC's and still the same result. Can it be the prop valve? TIA

[toolboxchev]

I went with a cheap reman about 10 years ago, kept blowing through Power Boosters about 6 years into it. The Power Boosters were a lifetime warranty so I got 3 replacements.

Finally put a decent MC on there last year and no more problems. The MC would do the same thing as the OP and excess fluid would go into the MC eventually bringing it to ruin also.

Lifetime Warranty products are worth it. Just got a new power steering gear box that lasted 10 years and new alternator.

[superfly66]

Originally Posted by toolboxchev

I went with a cheap reman about 10 years ago, kept blowing through Power Boosters about 6 years into it. The Power Boosters were a lifetime warranty so I got 3 replacements.

Finally put a decent MC on there last year and no more problems. The MC would do the same thing as the OP and excess fluid would go into the MC eventually bringing it to ruin also.

Lifetime Warranty products are worth it. Just got a new power steering gear box that lasted 10 years and new alternator.

Did you have the same issue as I had? Or just letting me know to go with new parts? I am not sure if the parts I had purchased were new or remanufacutured. It was from Toms Classics.

I bought the booster, MC, PV, all as a kit. Of course it was out of warranty by the time it went bad or I had know it went bad. And I contacted the company I bought it from and they told me to replace it with a 78 Corvette MC. Becuase that is what the set up was for. When I got one from the auto parts store it was exactly like the one I had minus, the plunger in the new MC was recessed. So, I had to make a 1" spacer for inside the new one. There wansnt anymore adjustment from the rod. Now I'm not sure which MC to buy unless I see it first. What model did you buy a MC set up for? Thanks for the reply!

[68Gold/white]

Originally Posted by superfly66

I know this is an old thread but, I'm having these issues as well. I put all new brake parts on front to back. Disc on front, all new lines, new MC, new Prop. valve, new rear drums parts, new booster. I read this thread a few weeks ago and replaced my already new master cylinder with a new one. I'm having the same issues. Back reservoir fluid moving to the front reservoir. 2 new MC's and still the same result. Can it be the prop valve? TIA

NO!
The master cyl is bad!!!
Get a NEW one, not a reman!!!

[superfly66]

Originally Posted by 68Gold/white

NO!
The master cyl is bad!!!
Get a NEW one, not a reman!!!

well maybe 3rd times a charm