Brake Problem

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sirhk100

Racer
Feb 24, 2003
16,232
146
63
Las Vegas
I'm having issues with the front brake system on our race truck. Components wise, it's a CNC Master Cylinder with a remote Reservoir. About 8" of steel braided line to hard lines down to the frame rails. Then about 18" of steel braided line out to stock factory Toyota Caliper running generic pads and some standard rotors.

5+ years of use 100% problem free.

Before the Mint this past March I replaced the front brake pads as they were getting low. When I did this, I did as I have always done on every brake system ever including this one previously... Pulled the caliper off... Used a C-clamp to compress the pistons into the calipers, install new pads, reinstall. When I did that I ended up with really spongy brakes that barely worked at all. Fought for a week straight leading up to the Mint bleeding the system using every method imaginable, replaced both the calipers, replaced the rotors, rebuilt the master cylinder, switched to different new pads, tried old pads again, etc... Eventually we got them to work.

On race day everything was fine but at one point in the race we smacked a rock which broke off a chunk of the inside bead of our front pass wheel which swung around and hit the caliper we think shearing off 1 of the 2 mounting bolts. We stopped, pulled the caliper and pinched off the line.

After the race got the broken bolt out, reinstalled the caliper and replaced the brake line that we vice gripped shut. Everything seemed fine. Testing went fine, no issues at all.

Fast foward to August for Vegas to Reno. 1 mile into the race that same pass front corner, the brakes ended up locking up. I had them simply crack the bleeder loose and retighten it to relieve the pressure and they took off. At mile 30 though they double barrel rolled the truck. Then at mile 80 with me driving the driver side front tire came off at about 40-50mph because of broken wheel studs. (they blew some beads when the truck rolled and I don't think they got the lugs fully tightened is my gut feeling) At that point we had no way of safely getting a tire back on with only having 2 decent wheel studs left in the hub we called it a day...

So after Vegas to reno I pulled that corner that had the wheel come off, cleaned it all up, inspected, reinstalled. Didn't replace anything brake system related though. After we cracked the bleeder at mile 1, we made it to mile 80 without issue brake related at least.

Took the truck out testing about 6 weeks ago. Had the fronts get hot from dragging and start to lock up again. Again, quick open and close of the bleeder and all good. Took it home, bled it all using a forced pressure setup on the reservoir. Went testing again, all seemed great.

This past weekend... Adjusted the brake setup to intentionally put some more slop in the brake pedal and lined up to start. This race had us stage in Pahrump Nev but we had a 15 mile police escort down the highway out to where we started. We made it about 12 miles on the highway and I could tell they were locking up so we stopped at released pressure. The whole race, every 10-30 miles we were having to stop and release pressure on them. We messed with the pedal adjustments a bunch and basically adjusted it to where the peddle was almost all slop with pressure just at the last bit to the floor basically. I ran the last 80+ miles without having to stop to bleed them but I could feel them occasionally starting to drag because the pedal becomes really firm when it's happening. We never actually had to stop to crack the bleeder but it was getting close for sure. We eventually had to throw in the towel 20 miles from the finish due to an unrelated failure but I still have no faith in the brakes right now.


I'm not sure where to start hunting this one down though? What would cause the system to not release all the brake pressure and cause the pads to drag and build heat causing them to lock up? From right now, the master cylinder was rebuilt less than 1000 miles ago. We don't run a residual pressure valve. Have never needed one, again 5+ years problem free until I changed pads. Since then I've fought issues ever since first starting with basically no brakes being really spongy which I basically replaced everything short of the brake lines trying to fix and now I've got them dragging and building heat it seems every 10-30 miles even with them adjusted to where the pedal is 80% slop.


Thoughts??? Is there possibly some sort of tiny port in the master that's plugged that should release pressure? I've had something similar happen on a dirt bike like 20 years ago that had a TINY pin hole in the master plug up that caused the same issue. I just had to get a spec of dirt out of that tiny port and problem solved. Anyone know if CNC master's have something similar?


Sorry for being super long but I figured the more info/details the better...

Thanks,
Khris
 
Last edited:

90_DangerRanger

Senior Member
Mar 17, 2017
251
9
18
San Marcos, CA
I've had these issues with CNC Masters. Did you call them and ask about what sizes you should be running for your calipers? Also, even though you only have 1000 miles on them, they might need to be re-built again which sucks but at least they are fast.

Side note, how full are you filling the reservoirs? they need more breathing room than you think... I learned that the hard way. Same thing, drug my rears and got them glowing before I stopped about 10 miles in haha
 

crazyracer

Tres Cuatro Tacos
Sep 27, 2006
2,647
107
63
Pacific Beach, CA
To me, it sounds like you still have air in the system.
As the fluid and air heats up, it should make the spongyness feel worse. The air expands and causes the brakes to drag more too. It's really common on motorcycles too.

If not that, it sounds like the caliper piston could be "notchy". When you put new pads in it now rides in a different neutral than before, and with more piston inside the caliper. Is there any way you can test a new caliper?
 

PSDirtracer

Senior Member
Oct 17, 2007
1,559
33
48
Bermuda Dunes, CA
You are using the same calipers from the start of racing it? That seems like the only part you haven’t rebuilt or replaced already right? If you are using stock Toyota ones can’t be too crazy expensive so it’s worth a shot to see if it helps.

Also have you done a full line flush and confirm you have flow though out the whole line system.
 

sirhk100

Racer
Feb 24, 2003
16,232
146
63
Las Vegas
I've had these issues with CNC Masters. Did you call them and ask about what sizes you should be running for your calipers? Also, even though you only have 1000 miles on them, they might need to be re-built again which sucks but at least they are fast.

Side note, how full are you filling the reservoirs? they need more breathing room than you think... I learned that the hard way. Same thing, drug my rears and got them glowing before I stopped about 10 miles in haha
When I initially built the truck I did call to discuss master cylinder bore sizes based on the calipers we were running. I haven't spoken to them since, that was like 6 years ago probably. Yeah, rebuild is easy, especially since when I rebuilt them earlier in the year I got frustrated and cut a large chunk of firewall out of the way to make access easier for me. LOL Typically I fill them up pretty full... I can try sucking a bit out and seeing if that makes a difference, I'm going to laugh if it's something as simple/stupid as that! It'll probably be a month or so until we have it in the dirt again but I'll add the idea to my list...

To me, it sounds like you still have air in the system.
As the fluid and air heats up, it should make the spongyness feel worse. The air expands and causes the brakes to drag more too. It's really common on motorcycles too.

If not that, it sounds like the caliper piston could be "notchy". When you put new pads in it now rides in a different neutral than before, and with more piston inside the caliper. Is there any way you can test a new caliper?
Yeah I guess I could probably con one of my friends that has the same vehicle these calipers are for into letting me install them on theirs to see if they work fine in the stock application still. That would at least put the calipers out of the list of potential issues...



buy new masters and new calipers as a part of prep and call it good?
LOL, been there done that, literally the two or 3 nights leading up to the mint this year. But other than that... whatever baller!!!


Is there a way you can confirm that the lines leading to that corner aren't obstructed? Maybe there is crap in the line that has turned into a one way valve?
I could disconnect both ends and send compressed air through I suppose? They do bleed easy though and when I pushed the pistons in the reservoirs definitely filled obviously...


You are using the same calipers from the start of racing it? That seems like the only part you haven’t rebuilt or replaced already right? If you are using stock Toyota ones can’t be too crazy expensive so it’s worth a shot to see if it helps.

Also have you done a full line flush and confirm you have flow though out the whole line system.
Haven't done a specific full flush but I'm going to now using a pressure system on the reservoirs to push the fluid through.

Same calipers from day one until just before the Mint this year. That's when I replaced pads and ended up spongy like 2-3 nights before the race. Up against a time constraint after spending hours bleeding with vacuums, pressures, pumping, gravity, raising/lowering ends, etc we started tossing parts at it and that's when we replaced the calipers. These calipers along with the master rebuild and rotors have less than 1000 miles on them. BTW, the spongy feeling leading up to the mint ended up being the rotors. They were worn at an angle so the old pads were worn into match the angle they wore at. When put new pads on with the old rotors we ended up finding that they were only touching at the most inner surface of the diameter and the spongyness was the pad/caliper trying to compensate for a rotor surface that wasn't parallel. Rotors were the last part we replaced once we starting measuring them and found them to be so tapered. Brakes were good at that point. We ran our shortened 2 lap mint for 230 miles problem free. Then tested for V2R problem free and at mile one at V2R is where they dragged and locked up for us the first time. released pressure, called it a day 80 miles later but they never locked up again. They dragged one time testing between V2R and this past race weekend. This weekend though was absurd. We cooked them a couple times... Smoking hot literally and neat colors on some of the parts now too...
 

Turboyota

Senior Member
Mar 29, 2017
2,764
745
113
Yucaipa
My money is on master cylinder. I have had similar issues with CNC master cylinders—cleaning and rebuild kits fixed it. After a couple frustrating/aggravating CNC issues i told myself the next build gets Tilton or Wilwood. (The condescending “tech help” lady at CNC may have biased my opinion, though)

Also, I had a very similar situation recently on my DD Toyota Camry. The car started shuddering like I had an unbalanced tire, then the top speed started dropping. Shuddering gradually increased. I pulled over and smoke was coming out of both front wheel wells, and both front hub caps were wicked hot. Pedal feel got progressively stiffer as I drove.

Replaced the master cylinder, problem solved.

I can feel a slight warp in the rotors now, but whatever...
 
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90_DangerRanger

Senior Member
Mar 17, 2017
251
9
18
San Marcos, CA
they go bad over time. the shit metal they use get pitted too and start to wear all the internal parts quicker. X2 on the Tiltons.... everyone i know told me not to bother and just switch teams haha!

Turbo, "he condescending “tech help” lady at CNC " i know who you are talking about all too well haha!
 
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