2 years chasing low pressure at the caliper...

[Icetech]

I posted this over on FBBO but thought i would try here also to get any ideas..

K.. my car is a 62 plymouth, converted from drum to disc over 2 years ago, have never had good brakes, by not good i mean barely slow down.

Here is the system layout..
7/8 Master cylinder (new this week)
all new lines (2 years ago)
everything new actually... kit was from pirate jacks..
Brass distro block with a wilwood prop valve on the rear line.
Pedal ratio is 7:1 with the pivot moved forward to stop it from going overcenter (trick we used on ffr cobras with manual brakes)

DOT5 Fluid.. (i have never used dot 5 but plenty of people have, i can't see it being the issue but never know)

So in the 2 years i have replaced the entire front system twice now other than lines.. 4 master cylinders (smaller sizes til the 7/8) 2 sets of rotors, 2 sets of calipers. 2 sets of pads, now using EBC yellowstuff

I eventually got a pressure gauge to try to track down the issue, with the 7/8MC i get 1000psi at the MC and 1200 if i really stomp it.. i get 1000psi at the connector where the flex line meets the hard line.
At the caliper i get 550-600 tops..

The brakes feel like the pads just won't grab the rotors which i am assuming is from the lower pressure, if not then i am really confused.. Rotors are brand new bendix as of a week ago, calipers are new also as of a week ago.

So.. the only thing between the 1000psi and 550psi is the flex line and caliper, and before someone asks.. not the caliper isn't upside down I have bled them many times, zero air, nice stiff pedal..

Right now i even plugged the passenger front port at the distro block to isolate the drivers side to try to find my pressure issue one just one before i re-add the other.

Soo.. i know flex lines can swell and make it so calipers won't retract, no issue with that here..

I get plenty of fluid out when when bleeding..

so i'm just hoping someone may have had an issue like this before or have an idea, parts canon hasn't worked. I'm to the point of looking for a set of drums to put back on just so i can drive it safely.

P.S. i have done brakes for 40 years and never ran into anything like this...

Gonna add a couple of vids showing the pressures.. the videos are me pushing at 3 different levels, normal braking, harder and very hard..

BTW.. the distro block one has about 3" of air in the line for the gauge and did 1000psi... caliper has zero air.

At Caliper

At distro block

 

[dano]

Can you check it at the hard line before the flex hose? That would isolate the flex hose or caliper. Flex line is braided? What caliper out of curiosity?

 

[Icetech]

Can you check it at the hard line before the flex hose? That would isolate the flex hose or caliper. Flex line is braided? What caliper out of curiosity?

I did test at that fitting and posted a video of the test above.. 1000psi...

Flex line is a braided from Drdiff

Calipers are brand new ones from rock auto, not rebuilds (2.5" piston which is a bummer but it didn't list the piston size) I wanted metal pistons and all the rest were plastic/phonelic or whatever....

 

[furyus2]

If you have isolated the problem to the flex line, and caliper, I would remove the flex line from the caliper, and then put the gauge on the flex line. The way things are made it could be the caliper. This test should show you

 

[Icetech]

If you have isolated the problem to the flex line, and caliper, I would remove the flex line from the caliper, and then put the gauge on the flex line. The way things are made it could be the caliper. This test should show you

Can't put the gauge on the flex line as it's a banjo fitting thought of that..

 

[furyus2]

If you have a fitting that will thread onto the banjo bolt, and if it will snug up with the hose , and copper washers, you could get a reading that way. Otherwise, you will have to throw some hoses at it, or rebuilt factory style calipers, whichever is cheaper or easier.
I know you don't want to throw parts, but it seem like you don't have a choice

 

[Icetech]

If you have a fitting that will thread onto the banjo bolt, and if it will snug up with the hose , and copper washers, you could get a reading that way. Otherwise, you will have to throw some hoses at it, or rebuilt factory style calipers, whichever is cheaper or easier.
I know you don't want to throw parts, but it seem like you don't have a choice

Calipers are 1 week old.... i have thrown all the parts at it I just don't see how the hoses could be doing it but not much else at this point

 

[furyus2]

I personally don't trust Chinese manufacturing, so that's why I suggested replacing the caliper with rebuilt original.

 

[toolmanmike]

Calipers are 1 week old.... i have thrown all the parts at it I just don't see how the hoses could be doing it but not much else at this point

And that's something you really can't test. It sounds like you are getting good pressure going into the hose but not much breaking power at the caliper. The calipers are working. I would search for a stock type rubber flex hose from a disc brake application and replace both. That's about all you can do to confirm or deny that the flex hoses are the issue.

 

[Icetech]

And that's something you really can't test. It sounds like you are getting good pressure going into the hose but not much breaking power at the caliper. The calipers are working. I would search for a stock type rubber flex hose from a disc brake application and replace both. That's about all you can do to confirm or deny that the flex hoses are the issue.

yeah.. i might just order 1 cheap rubber one to see if things change on that caliper... need to figure what car to order them for.. and measure the lenth of my current ones

 

[Icetech]

And that's something you really can't test. It sounds like you are getting good pressure going into the hose but not much breaking power at the caliper. The calipers are working. I would search for a stock type rubber flex hose from a disc brake application and replace both. That's about all you can do to confirm or deny that the flex hoses are the issue.

That was a good idea... just ordered dorman $10 line for a 73 satellite.. will be here wed. I was thinking of blowing another $100 on a pair of braided again.. glad i didn't

 

[Ricks70Duster340]

That was a good idea... just ordered dorman $10 line for a 72 satellite.. will be here wed. I was thinking of blowing another $100 on a pair of braided again.. glad i didn't

I'm very interested in what you find after you replace the braided line with OE style rubber.

 

[Dana67Dart]

My thoughts ...

1. The gauge at the caliper, if you press and hold the pedal 1-2 minutes does the pressure rise to 1000?
2. Post a photo of the distro valve
3. Post a photo of the hard line routing into and out of the distro valve.
4. Post a photo of the hard lines out of the master
5. You could take 2 banjo fittings and make this stack... Bolt-Copper washer-banjo- copper washer- banjo-copper washer -nut. Then measure out the end of the second banjo line. Just be sure the bolt is not too large and fills the entire hole of the banjo, same with the copper washers.
6. In one of your videos there is fluid leaking out can't pressure test with a leak.

 

[8formulas]

Icetech, I have the identical issue on my 70 Barracuda. So frustrating. You're not alone.

I just live with it.

 

[jos51700]

I did the 11 3/4" rotor conversion from drums on my Dart years ago and the pedal pressure was always mushy. I don't know if it was the wheel cylinders I used on the rear or what. Tagged for interest, and where did you get the the pressure gauge for the system?

 

[Mattax]

Could it be the restriction is in the caliper before at the bleeder location?
In which case the loss is pressure at the bleeder is tied to the fluid displacement. In other words the possibility that volume of fluid moved by the master isn't enough to move the caliper pistons.




Sucks that it was leaking.
It would be easier to compare if you could hold the pressure for a few seconds at say light braking and full braking efforts.
Light braking would also be like initial braking, whne all four brakes should be doing equal work.

 

[Icetech]

My thoughts ...

1. The gauge at the caliper, if you press and hold the pedal 1-2 minutes does the pressure rise to 1000?
2. Post a photo of the distro valve
3. Post a photo of the hard line routing into and out of the distro valve.
4. Post a photo of the hard lines out of the master
5. You could take 2 banjo fittings and make this stack... Bolt-Copper washer-banjo- copper washer- banjo-copper washer -nut. Then measure out the end of the second banjo line. Just be sure the bolt is not too large and fills the entire hole of the banjo, same with the copper washers.
6. In one of your videos there is fluid leaking out can't pressure test with a leak.

hahha i didn't even see the leak, didn't tighten it enough.. still got 1000psi though

I don't have pics with me.. (i'm at work) the front port is to the rear brakes, i removed all loops so no air could be trapped.. fronts and rears at the distro block are right (hard to mess up)

the only pic i have handy only shows the MC... going to test with a new line on wed..

 

[Icetech]

I did the 11 3/4" rotor conversion from drums on my Dart years ago and the pedal pressure was always mushy. I don't know if it was the wheel cylinders I used on the rear or what. Tagged for interest, and where did you get the the pressure gauge for the system?

Summit... i never needed one in the past but it's great for finding where a issue may be.. just gotta enjoy making up little lines to connect to like the MC if you are gonna check it

 

[Map63Vette]

I know someone at my old job had a similar odd problem, but it was with old rubber lines. What had happened was the rubber delaminated on the inside of the hose and when you applied pressure, a flap of the hose would peel off the wall and block the flow. Let off the pedal and it would flow again, so it would gravity bleed, but not take pressure. It was more or less acting like a one way check valve.

I don't think PTFE style lines would be able to do that though, so probably irrelevant here, but could be useful for others with a similar problem that find this thread.

 

[Icetech]

I know someone at my old job had a similar odd problem, but it was with old rubber lines. What had happened was the rubber delaminated on the inside of the hose and when you applied pressure, a flap of the hose would peel off the wall and block the flow. Let off the pedal and it would flow again, so it would gravity bleed, but not take pressure. It was more or less acting like a one way check valve.

I don't think PTFE style lines would be able to do that though, so probably irrelevant here, but could be useful for others with a similar problem that find this thread.

yeah, i'm up for trying anything at this point.... i literally have as much money into a stock setup as wilwoods would have cost now... it's insane...

 

[MobileCustoms]

Have you tried or can you try blocking flow to the rear brakes, maybe even bypassing the prop valve and going only to the front brakes, just to make sure everything up front is good (or bad)? Not sure if I saw but did you get a pressure reading from the rear brakes?
Can you temporarily bypass the flex lines for a test?

 

[1MeanA]

If there is no flow there should be no pressure loss. Pressure loss would due be to leakage (internal or external).

 

[Icetech]

Have you tried or can you try blocking flow to the rear brakes, maybe even bypassing the prop valve and going only to the front brakes, just to make sure everything up front is good (or bad)? Not sure if I saw but did you get a pressure reading from the rear brakes?
Can you temporarily bypass the flex lines for a test?

No prop valve it's a distro block, front and rear are totally separate circuits... ordered a new flex line to test, will be here wed.

 

[Icetech]

If there is no flow there should be no pressure loss. Pressure loss would due be to leakage (internal or external).

no leaks anywhere.. been in enough now that i would have noticed This is whats so frustrating.. a hydraulic system without air should have the same pressure from MC to caliper.. even if there is air a few pumps will compress it and it should at least come up some. I have bled these like 20 times with a power bleeder, normal friend helping.. and auto bleeders also (which i love)

 

[1MeanA]

Is this a factory master cylinder/caliper combo? Are you sure they are matched (volume)? It will be interesting to see what the problem was.