Technical Brake system help, please

Alright yall. Yep, I am still working on this. Sorry for the ‘dead zone’ intervals. I’m still dealing with cancer and sometimes I just don’t feel like fooling with it.

Anyway, check my logic: If the left OR right front was holding pressure, I could understand a possible bad hose OR if it were only the 2 rear wheels, that hose could be bad. But to think that ALL 3 rubber hoses picked the same time to start messing up…I just don’t see it.

So, I’m looking for something that controls all 4 wheels and there are only 2 suspects: The power brake booster OR the Master Cylinder.

So, here’s the plan, I am going to disconnect and plug the vacuum line to the power brake booster; drive the car stop and go for enough time to create the drag I was experiencing. IF it does NOT start holding pressure, I will replace the booster.

However, If it still starts holding pressure, the only thing controlling the whole system at this point would be the Master Cylinder, and I will replace it.

What says the brain trust?

 

Do the cheapest things first, again, the master is real old. That is what I would replace. Gut the old one after the dust settles and see what you find.

 

In my mind, the 'cheapest first' would be to disable the booster and road test, so that is my focus.
Good advice tho- thank you.

 

Pulled and plugged brake booster vacuum line - went driving around the neighborhood and brought it back home.
Once in the shop, It would not roll as easy as normal; therefore, pressure is starting to build.
Bottom line, the booster is not the issue.
My problem lies inside the Master Cylinder.
I will be changing it.
I finally found an identical M/C and bought it.
Might take me a couple of weeks, but I'll be back with further updates.

 

Did you check pushrod adjustment?

 

Also with the cap off the master do you get a little spurt up as you step on the brake? If
Not the vent ports are plugged. It will be one of those two things, both easy. However I prefer to fix things as opposed to buying parts but not everyone does.

 

Maybe stop light switch is pushing on the pedal arm. Happened a lot in the factory.

 

I'm having the same issue with a new Speedway 7" dual booster pedal assembly. I have been thinking I ended up with a drum/ drum master cylinder rather than a disc/disc with the 10psi check valves rather than 2 psi, but I just checked per 57 Fargo's comment and no spurt so I'll check pushrod length tomorrow.
Everything is new from rotors to master cylinder. Still on break in oil at 160 miles.

 

While I had the same issues that Thunder Road has my problem has hopefully been resolved. Still needs the test drive.
To be fair there is a utube video from Speedway that cautions that the dual booster is not adjusted from the factory. So I loosened the two nuts that hold the master cylinder and it developed a 1/8" gap from the booster with diaphram spring pressure. Then I removed the MC and with the pedal depressed was able to turn in the the adjustment bolt slightly more than the gap. The test was the spurting in both front and rear reservoirs when the MC was mounted.

 

I had previously checked pushrod length and it was ok,
but where are these 'vent holes' you mention? How would I get to them?

 

Just look down your master cylinder while someone pushes the brake, you should see little spurts of fluid. If your fluid was low they would actually break the surface.

 

This, there will be two holes in each section of reservoir, the vent ports will be the smaller of the two as well as the front one in each chamber, as said step on the brakes see if you get a little spurt out of both, if not clean them out.

 

How did you check the length? For fun shorten it a little if it’s adjustable or put a washer behind the master and see what happens. The cup seals are generally about 0.030” of an inch from the vent ports, if they don’t return back past the ports the brakes will gradually come on as the heat up

 

I once purchased a new master cylinder, i don't remember from who but it had a good amount of metal chips in it. Luckily I found this before installing it.

 

IIRC, I unbolted the M/C and pulled it forward enough to measure the pushrod length and then the depth of the hole it goes into. Measurements looked good- no interference or too long of a rod vs the depth of the hole. Like I said, this is a new condition that was not seen for over 6,000 miles of driving the car.

 

There should be a slight amount of play in it.

 

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Do you mean a BOOSTER/MC ASSEMBLY?

 

Oh, that's easy to explain.

Most likely made in ONE-HUNG-LO, CHINA from melted down SHERMAN TANKS salvaged during/after the KOREAN CONFLICT. That's AMERICAN STEEL and it is tough brother. It really hangs on.

 

You could try cracking the line at the MC to see if it is holding any pressure. If no fluid squirts out, you can pretty much eliminate the MC as part of the problem. It may ooze a bit, but if it squirts out under pressure, then it is a fluid flow problem at the MC.
Another potential cause would be the caliper pistons sticking or the pads hanging up from dust build-up. But that might favor one or the other.

 

This is my problem.
I removed the lid and stepped on the brake many times and never got ANY movement of the fluid. NONE.
So, unplugging the vents is next on my list. Thank you 57 Fargo.

 

Good morning guys in reference to the posters problem. On my son’s OT car I replaced everything brake wise. Took it for a ride and the brakes on the front started to drag and then started to smoke ( I was on the Interstate) I waited a while and it cooled down so I drove it home slowly . I went round and round with the place I purchased the parts from replaced them twice and the same issue, calipers rotors and hoses. Same thing. I was at a point I was just going to burn it. I was getting desperate, my son was in College and really needed his car he was working a job away from the College. I was beside myself, I was and still am a mechanic with many years experience so I am also embarrassed. At the last moment I called Brakes are us and was able to talk to an employee that I could have hugged when everything was said and done . He asked me if I checked the master cylinder I said I replaced it twice checked the depth and everything, He asked me if the master was leaking and then I remembered a leak from up that way. He explained that if the master was leaking out the back and got into the booster between the vacuum and and heat the rake fluid expands and will lock up the brakes. I replaced the booster and that was the problem. Sorry for the long rant . Have a great day. Ron.

 

Nothing is ever simple is it?

So, I installed a new Master Cylinder after bench bleeding it.

Then I bled the front calipers and drove it a bit to see if it tightened up again and it did NOT. YAY, it’s fixed. All there is left is to bleed the rear brakes.

So,I did that today and backed her out for a test drive. WHOA, what the heck happened to my brakes?? The pedal made a little tension and then started going to the floor – pumping them up did not work.

Back into the shop she went. Checked the brake fluid level and NO fluid is missing. As fast as it was going to the floor, fluid would have been just gushing out…but it is not.

It seems to me that I got a junk “new” Master Cylinder.

Thoughts and opinions??

 

Master cylinder bore size? Compared to the old one?

 

same size - 1 inch

 

The master is bypassing internally. Some do it on a big scale like yours, others we had to chase around to duplicate the problem.

 

Last post on this one - M/C has been changed again and this one works correctly.
All problems have been resolved.
Thank you to all who helped...much appreciated

 

Those that bypassed were either blatant offenders that did it constantly, or we had to "tickle" them into doing it for us. Playing with the brake pedal usually did it in the shop. I found it to be a black art in some cases.

 

I got a bum steer from Speedway once. bench bleeding got returns on one side only. They sent a new one .