Help brake issues

[Gus]

I have a speedway master cylinder for disc brakes no boost, running 4 wheel disc with no pressure valves.

Why are the bakes holding pressure , so much that it stops abruptly when you let off the gas and sitting on a slope it wont even roll , I know someone else that is experiencing the same problem ??? using the same master cylinder .

It has been torn apart cleaned checked ...

It's not always, it's like it has to build up, my friend has to pull over about every 40-50 miles and release the pressure ??????

Mine was fine when I parked it about 3 months ago but now I fired it up drove it 1 block and it was so bad the back tires were trying to spin because the front brakes were locked down so tight...???? Anyone have a answer ???

 

[Dirtyrat]

Is the pedal returning all the way? And is the brake MC rod adjusted for some free play?
Thats about all I can think of causing it...

 

[junkdealer2]

Whew, this description brings back bad memories. If the rubber brake lines aren't new, replace them. A common problem is the collapse of the liner in the rubber lines,, they will pass fluid to the wheels and hold pressure on the cylinder or caliper. The lines dont have to be very old and even hanging a caliper from the new line can cause this collapse to occur. Mixing brake fluids, using old brake fluids, using a fluid that is not compatable with the hose, and cleaning old lines improperly can cause the collapse to happen. This problem has reared is head many times and even happened on a forklift once.

 

[sgtpontiac]

This would be my guess as well...

Whew, this description brings back bad memories. If the rubber brake lines aren't new, replace them. A common problem is the collapse of the liner in the rubber lines,, they will pass fluid to the wheels and hold pressure on the cylinder or caliper. The lines dont have to be very old and even hanging a caliper from the new line can cause this collapse to occur. Mixing brake fluids, using old brake fluids, using a fluid that is not compatable with the hose, and cleaning old lines improperly can cause the collapse to happen. This problem has reared is head many times and even happened on a forklift once.

I think junkdealer has hit the nail on the head......

 

[Gus]

All new s.s lines , new hose ???? 4 brand new calipers all vacuum bled ??

Dirtyrat I need to check the play and see how much is there on the plunger rod , I may need to put a return spring somewhere to help pull it all back ???

I don't know I'm at a loss ???

 

[sgtpontiac]

Gus....back to basic's....

All new s.s lines , new hose ???? 4 brand new calipers all vacuum bled ??

Dirtyrat I need to check the play and see how much is there on the plunger rod , I may need to put a return spring somewhere to help pull it all back ???

I don't know I'm at a loss ???

If it was ok 3 months ago...what has changed?? If it's only the front brakes there has to be a simple reason...don't over think it...(like we all do sometimes)...MC, two calipers, two brake hoses and the lines to them....anything get pinched, smashed or bent? I've seen new hoses collapse internally made wrong..but normally it would only affect one wheel not both.....is it both wheels or just one? If both has to be something from the MC or lines to the wheels....other than that...I'm stumped....[S

 

[Gus]

Well let me ask this , the MC is backwards the plunger is facing forward. So since it's backwards , it don't matter which chambers feeds which axel , what is normally the front chamber is feeding the rear and what is the rear chamber is feeding the front ?

And it is only both front wheels not the rear locking down ???..

The other guy I know , his can be sitting for days no one near it and he will notice his brake lights are on and some how it built pressure ???? His is the same only the fronts lock down not the rear everything is the same all new lines , hose , calipers , master cylinder ???

And I hear there is a third car doing this also ????

 

[Gus]

I think I may just go buy a old Maverick sand the paint off and put four different wheel on it and bust out the side glass and call it MY RAT [cl

 

[dmw56]

It's the aliens doing it. This is New Mexico you know.

 

[Sniper]

As DR has already mentioned, I would look at the push rod and see if there is any slack in it, if it's to long it will be applying the brakes just like pushing on the pedal.

As far as the brake lights coming on with your friends car. I can bet he has an inline hydraulic brake switch. After it sits, the rear brake (drum/shoe) springs on the shoes will pull in and exhaust the fluid out of the slave cylinders pressuring up the system causing the switch to activate turning on the brake lights. The only fix I have needed to over come this problem was to set up the brakes with less clearance between the shoes and drums. It just stops the need for such a long travel for the brakes to work and so in turn, the springs pulling the shoes back won't push as much fluid back into the system, causing the switch to trip. Now if it's disc brakes back there, I don't know what's causing it without seeing the whole set up. Crystal ball is a bit foggy at this distance.

 

[sgtpontiac]

Hey another question / comment

Is your buddies MC or yours lower than or at the same level as the calipers / lines? could it be back filling and causing pressure? How high is level of fluid in the MC after the brakes are locking up....could it be back filling and pressurizing from the rear brakes to the front?...since all fluid expands with heat, there has to be somewhere for it to move if it does...if the MC is full then it will pressurize one if not both of the reseviors (spelling)
Have either of you popped just the MC top off when the brakes are locked up in front rather than opening the bleeder screws? I'd be looking in that direction....not sure that which resevior you use would be the issue....Just a thought...

 

[Dirtyrat]

I still think its a travel issue, at the stop of the pedal on return, the MC has not returned the actuator rod fully, keeping pressure on the lines... I don't know what else really could cause this.

I'm glad you mentioned the rear lines going to the front, I kinda forgot all about that... I'll have to see if they are labeled on my MC that way...

Anyhow, let us know what you find out!

 

[junkdealer2]

Also, brake fluid rating can cause problems. What DOT rating is the fluid your using, #3, #4, #5? Some brake hoses and rubber parts are rated for DOT 3 only. When exposed to DOT 5 these parts deteriorate quickly and fail. If the pressure can only be relieved by cracking the bleeders then I would replace the rubber lines after clearing the system of all old fluid. I had a new line fail in weeks when the owner had used DOT 5 to top off a DOT 3 system. DOT 5 will cause dot 3 rated rubber to swell.
Seperate question: is the master cylinder mounted above( ie firewall) or below( frame) the wheel cylinders?

 

[sgtpontiac]

The MC height was what I was driving at...

back flow into the MC creating pressure to the point that the fluid heats and pressure builds as it's heated....
Ok..since we are not talking about a boosted system...(power brakes)...the return of the pedal would be limited to either an assisted pedal return spring (which you seldom see) or the MC itself.....I would assume that the pedal return is probably limited to the MC piston spring and residual pressure....correct? Again, since it worked before...what changed.....Unless there is an assist spring of some type added....?? I am guessing that binding of the brake pedal assembly has already been checked??? So if the MC wasn't returning all the way, that should be easily diagnosed by simply pulling up on the pedal and seeing if it releases the pressure to the brakes... Since none of us know how the system is set up....it is difficult to diagnose...but with two vehicles doing similar stuff, there is probably a common cause and common cure......too much of a coincidence but not impossible...hate to overthink this stuff....cause it's basic hydraulics.....not rocket science...

 

[Gus]

The MC is about 6 to 8" below the front calipers at least 13/17" below the rears.

The last thing I did was fill my MC when I parked it 3 months ago because that was when I "THOUGHT" I was done working on the brakes, all was good then.

SOOO I'll dip a small amount out check the free play which there may not be much there because I wanted instant PEDAL

And yes his switch is in line and so is mine, but mine is not hooked up yet.

Should I run the residual valves ????

 

[Dirtyrat]

You need it if your MC is below your calipers.
This is one of the best reference pages for this:
MPC Brakes

 

[gold03]

Pull the rubber brake lines off and check them for free flow both directions with compressed air.

Gold03

 

[Gus]

You need it if your MC is below your calipers.
This is one of the best reference pages for this:
MPC Brakes

I went and looked at that and read all there was so, I guess I'm not done with the brakes ???

 

[Gus]

Pull the rubber brake lines off and check them for free flow both directions with compressed air.

Gold03

I've did that ????

 

[Gus]

DR ..I've got more work to do I guess [S [