Wierd brake problem

[DozerII]

In the 2200 miles I have put on the Essex, three times while sitting in gear stopped the brake pedal has gone to the floor. One pump and it's hard and back to normal.
Front disc, rear drum, manual corvette style master cylinder 7/8 bore, all new parts. I'm using the stock 1975 Caprice proportioning valve. I have checked the entire system front to back and there are no leaks anywhere, rear shoes are properly adjusted.

Anyone have any ideas on what's happening, I have only ever run into this with power brakes and a bad booster??? [S

 

[Old Iron]

Your master cylinder is going bad, no matter how new it is, it's giving up.

 

[Oldphart]

Your master cylinder is going bad, no matter how new it is, it's giving up.

I agree, master cylinder. In my experience a bad booster resulted in a hard but ineffective pedal kinda like when the engine isn't running.

 

[DozerII]

Your master cylinder is going bad, no matter how new it is, it's giving up.

I agree, master cylinder. In my experience a bad booster resulted in a hard but ineffective pedal kinda like when the engine isn't running.

Okay so it must be by-passing once in a while!! Better get a new one coming

Thanks

 

[Neto]

Could there be small bits of rust or grit in the fluid that is sometimes allowing the fluid to flow back through?

 

[DozerII]

Could there be small bits of rust or grit in the fluid that is sometimes allowing the fluid to flow back through?

With all new lines and components, plus the all the fluid is only a year old I don't know where the rust would come from. [S

 

[smallfoot]

Quality control is almost non-existent anymore. New doesn't mean much if contaminants have reached a new item while being stored or handled. Sometimes we're our own enemies by not taking proper care while those parts are laying around the shop, waiting to be installed. Could be just a little trash in the fluid.

 

[DozerII]

Quality control is almost non-existent anymore. New doesn't mean much if contaminants have reached a new item while being stored or handled. Sometimes we're our own enemies by not taking proper care while those parts are laying around the shop, waiting to be installed. Could be just a little trash in the fluid.

Good point, but I did take the parts right out of the boxes and installed them. I also blew out all the new brake lines. I guess I could try a complete flush of the system and see if it does it again?

 

[offroadrolls]

Shelf rot. I've seen it a dozen times on master cylinders over the years. NOS or obsolete stuff. I got in the habit of pulling both masters and wheel cylinders apart to at least wipe things off and take a look.

 

[DozerII]

Not sure if it was shelf rot or not, but while my brother was here for a visit this past weekend we emptied the master and flushed the system. It sure wasn't the clear clean looking brake fluid I expected to se come out in my catch bottle[S

 

[skull]

mine

the master cylinder in the dodge truck was brand new, it went bad with no warning, smucked a rig, internal failure was the cause, put in a new oe, cheap insurance.

Later

 

[53 stickfigure]

I had a perfect pedal but holding it for about a minute it would go to the floor. It got so that after it went all the way down the brakes wouldnt let go and the car move until the pressure would bleed off from the master cylinder.

 

[DozerII]

Okay guys, thanks for all the comments, ordered a new master cylinder and got it installed . Everything worked prefect for approximatley 60 miles, then today I pulled up in front of the shop and the pedal went to the floor again, the brake warning light came on. I pumped the pedal once then a second time and the pedal came back and the light went out. The only parts left in the system that are not new are the hoses and the proportioning valve. The hoses all looked good and were not hard or cracked, could the problem be in the prop valve?[S

 

[kenny c]

Could it be leaking by internaly in the proportioning valve? it might be worth changing.

 

[dutch]

not sure how the propvalve would cause something like that... [S
I`m thinking bad mc. I`ve had my share of bad "new" stuff...

maybe a soft hose expanding under pressure?

 

[DozerII]

Could it be leaking by internaly in the proportioning valve? it might be worth changing.

not sure how the propvalve would cause something like that... [S
I`m thinking bad mc. I`ve had my share of bad "new" stuff...

maybe a soft hose expanding under pressure?

Thanks guys, If I understand the prop valve it just has a plunger that slides one way or the other if there is a failure front or rear as well as the proportioning function.

I think I should order new hoses[S[S

 

[smallfoot]

Man, I'm trying to remember what post it was, Didn't somebody else here in the not too long ago past do a post on a similar problem where the op found a problem with the length of the actuater rod on the mc was adjusted either too long or short and the mc couldn't work right until the rod throw was adjusted right???

 

[dutch]

Man, I'm trying to remember what post it was, Didn't somebody else here in the not too long ago past do a post on a similar problem where the op found a problem with the length of the actuater rod on the mc was adjusted either too long or short and the mc couldn't work right until the rod throw was adjusted right???

yea, but why work fine for 60mls...?

 

[DozerII]

yea, but why work fine for 60mls...?

And the first worked fine for 2100 miles[S

 

[smallfoot]

Yeah, I know...trying to make some sense of it. There was a post about that rod not being adjusted right and the plunger not making it by the vent every time or something...just trying to figure out the "why not all the time" deal.