Diagnosis help needed - weird brake pedal issue

[bkahler]

Background:

 

Rusty Hope front disc brake conversion. Rear wheel disc brakes with the Cherokee differential conversion.  97 Cherokee master cylinder.  Silicone brake fluid.  All new brake lines and rubber hoses.

 

Problem:

 

After truck has been sitting for a few minutes or more the brake pedal will go all the way to the floor but you can pump the pedal multiple times and it will build up pressure and it becomes reasonably firm.  However, if you keep pressing on the pedal it very gradually goes to the floor.  Once it does that you can pump the pedal back up and you can repeat the process again and again.

 

Diagnostics so far:

 

Verified all lines are tight.  No sign of fluid anywhere and the MC reservoir is not losing any fluid.  Initial research via google was the MC may not like the silicone fluid and it might be bleeding by the seals.  However I don't think that's the case because of the results of the following test.  One thing I found online was a suggestion to squeeze off the rubber brake lines and repeat the test each time to see if the problem can be pin pointed to an individual wheel.  So I did this by putting a clamp on the rear hose to the differential and the results were still the same.  So I then moved to the driver front wheel and again clamped the hose shut.  No difference, pedal still went to the floor.  

 

Next I did the right front but the results were different.  The pedal did NOT go to the floor and on the first pump there was reasonable pedal resistance.  I pumped it a few times and the pedal stayed reasonably firm.  I then waited about 15 minutes and tried the pedal again and it was still reasonably stiff on the first pump.  So, I left things as they were for 24 hours and tried the pedal again and it was still reasonably firm.  

 

At this point I removed the clamp on the right front wheel and on the first pump the pedal immediately went to the floor and I had to pump it up to get a firm pedal.  I then waited a few minutes and tried again, the pedal went to the floor.  It would appear the problem has to do with the right front wheel caliper or hose or...?

 

Nest step:

 

At this point I'm not sure what to do next other than try replacing the caliper.  Before I do that I thought I'd check with all Y'all to see if there were any other possible sources of the problem?  

 

What baffles me is what could be happening inside the caliper to cause these symptoms?

 

Thanks!

 

Brad

 

 

[kencombs]

Wild guess:   There is air trapped in the caliper.  Take a look at the bleed screw location on the  calipers.  On some it is possible to install with the bleed too low allowing air to be stuck in there.   Usually due to calipers being installed on the wrong side, when they are not ambidextrous, or whatever you call those that fit only one side in the original installation.

[Bryan]

That's what I was about to say.  Some come in left and right specific parts.  Bleed fitting should be near the top.  Also read something about hose clamps sometimes being too tight crimp the hose and cause problems.

[bkahler]

Thanks for the replies.  Bleed nipples point straight up.  

 

I tend to agree that it has to be air in the caliper which baffles me because I bleed until no bubbles and I did both front calipers the same way.  The drivers side seems to ok yet the passenger side doesn't.  Just seems weird to me.

 

I bought a pressure bleeder and once I get a rubber gasket made for the cap that goes on the MC I plan on trying it to see if that makes a difference.  I know silicone is hard to get all of the bubbles out but I've done several other vehicles with silicone and didn't have an issue like this.

 

As for hose clamps, there aren't any on the rubber hoses so that possibility shouldn't exist.

 

Of course there is always the possibility that I THOUGHT I bled the right front caliper but didn't 

 

[Jerry Roberts]

" Of course there is always the possibility that I THOUGHT I bled the right front caliper but didn't  "   So bleed it once more just to make sure . 

 

[bkahler]

Yeah, another bleeding is going to be the next thing that happens to try and resolve the problem.  

[Tooljunkie]

A few things come to mind, loose wheel bearing is one. If not, remove wheel and watch for movement as someone presses pedal. You didint mention if you had a proportioning valve,as there needs to be a residual valve to hold slight pressure at each wheel. 
i would check for binding brake pads in the mount as well. A hanging pad will flex, and when pedal is up,it will relax and push piston back into caliper. Everything needs to slide freely. Guide pins too. 

[Plymouthy Adams]

do  not rule out master cylinder internal bypassing which is the key symptom you describe with the light pedal pressure and pedal going to the floor...if you pump and hold really hard pressure here and it stay up...confirmed...!  

[bkahler]

1 hour ago, Tooljunkie said:

A few things come to mind, loose wheel bearing is one. If not, remove wheel and watch for movement as someone presses pedal. You didint mention if you had a proportioning valve,as there needs to be a residual valve to hold slight pressure at each wheel. 
i would check for binding brake pads in the mount as well. A hanging pad will flex, and when pedal is up,it will relax and push piston back into caliper. Everything needs to slide freely. Guide pins too. 

 

Everything is new and I've not yet driven the truck.  However I'll pull the wheels and check to ensure nothing is sticking.

 

 

[bkahler]

1 hour ago, Plymouthy Adams said:

do  not rule out master cylinder internal bypassing which is the key symptom you describe with the light pedal pressure and pedal going to the floor...if you pump and hold really hard pressure here and it stay up...confirmed...!  

 

I've believe the master cylinder has been ruled out due to the test results that I got.  The pedal stayed firm once I clamped off the right front brake hose.

 

 

[Plymouthy Adams]

good deal.....nothing beats testing each item for sure.....

[JBNeal]

Is it possible to switch sides on them front hoses?  Maybe one is expanding and contracting a little bit.

[bkahler]

9 hours ago, JBNeal said:

Is it possible to switch sides on them front hoses?  Maybe one is expanding and contracting a little bit.

 

Yes, it's possible but I would rather not have to do that unless I know for certain I have to break into the hydraulics.  

 

The hoses are brand new so if one of them failed this early then that doesn't bode well for the rest of the truck.

 

I'm going to start with doing another bleed, not sure if it will be pressure or pedal.  Pressure bleeding I can do myself otherwise I'll have to track someone down to help pump the pedal.

 

If bleeding doesn't solve the problem then I'll pull the wheels and make sure everything is free and lubed probably.  

 

[Tooljunkie]

I pressure bleed. I have a brake hold tool i made. Depress pedal and lock it. Crack bleeder and repeat. I will say, it not the hose. However, clamping a brake hose can damage it. 

[Dodgeb4ya]

I used to use a spring loaded telescopic rod to bleed brakes long ago...worked good.

Now days mostly use a steel pressure tank bleeder..or a Vacula  bleeder.

[bkahler]

11 hours ago, Dodgeb4ya said:

I used to use a spring loaded telescopic rod to bleed brakes long ago...worked good.

Now days mostly use a steel pressure tank bleeder..or a Vacula  bleeder.

 

I've used various props for holding the pedal down over the years, unfortunately I don't have the seat in yet which makes using some sort of prop a little problematic.  I've never done pressure bleeding before but I bought one just recently with the hopes of solving this problem.

 

[P15-D24]

You have air in the lines or a leak letting air get in. Pressure bleed and it that doesn't fix it pressurize the system and wait a few minutes. Then check every fitting and junction for leaks. 

[Tooljunkie]

14 hours ago, P15-D24 said:

You have air in the lines or a leak letting air get in. Pressure bleed and it that doesn't fix it pressurize the system and wait a few minutes. Then check every fitting and junction for leaks. 

Could be a very small leak, letting air in. Helper required. Abd a seat of some sort, have to push hard on pedal. 

[P15-D24]

5 hours ago, Tooljunkie said:

Could be a very small leak, letting air in. Helper required. Abd a seat of some sort, have to push hard on pedal. 

I had one on my B1 which was a hairline crack in a junction block, took forever to find.

[Tooljunkie]

I sit in seat put both feet on pedal and push. I call it a stress test. It better hold. If there is the slightest leak i will notice. 

[Sniper]

On 3/13/2022 at 10:06 AM, Tooljunkie said:

there needs to be a residual valve to hold slight pressure at each wheel.

 

Actually, there doesn't necessarily NEED to be a residual pressure valve at all. 

 

Why are there residual pressure valves?  Back in the day, before residual pressure valves, wheel cylinder cup seals would tend to slowly leak by when not in use and cause all sorts of issues from contaminated linings to no fluid in the master cylinder.  It was found that putting a slight bit of constant pressure in the system kept the cups in the wheel cylinders expanded and stopped the leak.  So residual pressure valves became the norm in a drum system until the OEM's started putting expanders in the cup seals and did away with RPV's in the master cylinder.  Contrary to what some brake websites will tell you they are not there to counter the brake spring pressure in the drums, if that were so you'd have dragging drums brakes and would be unable to set the clearances.

 

Disc brake calipers don't have cup seals nor did they have the issue of leakage so no OEM residual pressure valves there.    

 

Then some hot rodders started putting disc's on cars that had drums and master cylinders under the floor board.  They found that they had to sometimes pump the pedal on the first drive of the day because it was suspected gravity would want to transfer the fluid out of the calipers into the master cylinder.  How that applied to only calipers and not wheel cylinders I don't know.  A 2psi RPV was the "fix" for the discs, the drums still needed one regardless.  This causes the discs to drag all the time, not much drag but there nonetheless.  The proper fix would be to put the master cylinder up higher and let gravity work for you , not against you.

 

Anyway, the "need" for RPV's is a bit of tribal knowledge that gets tossed out without the technical knowledge as to why it was done and if it needs doing in your case.  I am wrapping up my disc brake swap and need to pull the RPV out of my stock master cylinder, because it will hold the calipers tight for at least 24 hours, lol.  I will put in an RPV in the line to the rear drums and test from there.