chevy p-30 brakes

[lonfu]

Had to replace my front rotors and pads and master cylinder. Bled all wheels and all working fine. Then they started dropping again to the floor. I noticed last trip after a hard climb through the nevada mountains at the top of the pass that the rear brakes were non existant and the fronts were grabbing hard at the bottom of the pedal travel. I had to stop and let the moho cool down so I boondocked at the top of the pass. Next day going down the hill all was good again. As the engine heated up my brake function started to reduce effectiveness, especially the rear disc pads.

When I got home I pulled the wheels and the rear pads and calipers were working correctly. Traced the brake lines and found that they run parallel to the exhaust manifold for about 6 inches where they connect to the master cylinder. The brake lines for the 6 inches are only 1 7/8 inches from exhaust manifold. The fluid was boiling before it was even entering the ABS system. Of course the DOT3 was boiling and then cooling causing water vapor to form in the brake lines and mastercylinder.

This a very serious manufacturing defect that Chevy refuses to address. To repair, I made a heat shield from some light steel to protect the brake lines and master cylinder. I bolted it on to the main frame channel. Then I've replaced the brake fluid with DOT5.1 rather than DOT3 as the shop manuel states. The boiling point of DOT5.1 is much higher than the DOT3. Don't bother with DOT4.

In order to correct this engineering mistake, you must do both, heat shield and upgrade to DOT5.1, just changing to the more expensive fluid will not solve the problem, it will just put off failure for a little longer. Do not use DOT5, it will not mix with the DOT3 or DOT4.

In searching the net I found very little solutions for this very dangerous and common problem. Apparently chevy was aware of this problem when they sold to workhorse. No recalls have been made at this time to correct this problem. Failed front rotors are a common symptom of the boiling brake fluid problem due to the fact that the rear brakes are disabled by the hot gas in the rear line so all the stopping responsibility is thrown on the front rotors and pads. Then the over heated boiling brake fluid freezes the front rotors and "wallah" no brakes at all!!!

[lonfu]

It would be nice if I could add my own photo of what it looks like. For some reason they have the browse and download a pix feature turned off on this forum format.

enblethen May I suggest that you get a heat gun at harbor freight, and after a climb up a 15 mile hill, measure the temp of your exhaust manifold. It is a normal function of the manifolds to get red hot under full throttle up a hill at 7000 plus feet. After 100k or so they usually crack and have to be replaced. It is normal!

[BrianinMichigan]

Can you mount a heat shield between the two points? Edit: Opps just the the comment. I added a heat shield between the headers and the starter on mine.

[Chopperbob]

In addition to the work in previous post I also installed
a cold air intake and a Magnaflow 3" straight thru muffler
to try and reduce heat.
Bob

[Chopperbob]

I installed two 3" air ducts from the bumper to the manifolds
with à blower in each one. No big deal.
I just installed four new brake lines, flushed 3 qts
Of fluid. All this helps but they are lousy to start with.
1997 Safari Trek. I try to use engine brakeing as much
as possible.
Bob

[enblethen]

You need to find out why the exhaust is getting red hot. Are they stock manifolds? Are you trying to go up the grades at full speed? You may needs to adjust your driving habits!
I would believe that heat builds up in the rubber lines closing them off. This is common.

[lonfu]

Ok, had a chance to make a run and problem is solved. I can hear the rear brakes grabbing the discs when I'm moving.

Glac, Measure the distance from your brake lines from your master cylinder to the closest point on your exhaust manifold. Mine was 1 7/8 inches. Some are closer than others. When I say they were hot to the touch I mean they burned my fingers when I touched them!

bsinmich, yes, completely replaced the fluid in the system each time. Pulled the fluid out of the Mastercylinder (mc) then refilled with new DOT 5.1, then bleed until all air and clear fluid out the rear pass caliper first, then rear drive caliper and so on.

enblethen, they are not weak. The rear calipers would stop functioning when I would go up a grade due to boiling brake fluid in the rear lines.

Like I mentioned my exhaust manifolds get red hot when going up a steep grade towing. At the top I noticed that my front end would dive when I hit the brakes and they were much lower than when I headed up the hill. This is not a normal function of a braking system.

I was finally able to figure out how to add pix, web site needs new software. drag and drop...... Problem solved, no more boiling brake fluid....

[bsinmich]

You stated you bled the brakes but did you flush all of the old fluid out and add fresh fluid?

[BULLETLS1]

I had a problem that when the master cyl was installed (new), apparently the pedal linkage was a little tight (definetelly had clearance for free play COLD), BUT shen heated the gap was filled and the very little hole in the bottom of the tank in the master cyl was restricted, hence freezing the front brakes, in which produced more heat, more restriction, and LOCKED UP front brakes! I was able to pull over and oppened the front calippers bleed screws, one of them let go with a squirt and unlocked the RV.
I called my mechanic to protest about the brake job, he said that that particular caliper design gets very easilly "ovaled" and grabs the piston (I understand that you did NOT changed or serviced the calipers), after that poor explanation, I had to continue my trip, the brakes had cooled by then, I increased the free play of the pedal and the hydroboost, and make shure the rod between the hydrob, and the main cyl was IN THE INTENTED SOCKET, Not kinked or resting on the piston edge of the main cyl... ans sure enough' after a few blocks I started to feel brakes grabbing again, in desesperation (and anger perhaps) I pumped hard and rapiddly the pedal, suddenly, the front brakes RELEASED With a CLANK!!!, (ohhh-s!), but the brakes DID work as supposed!!!!!, few miles ahead, same grabbing feeling, pumped the brakes, fainter and easier clank.... worked as supposed..... one last time few miles ahead, and released again w/o noticeable clank, and never again in a trip >1200mi, perhaps some rust in the piston area, and USE had "fixed" the issue!!!

[Gjac]

I have a 95 P-30 chassis. Are you saying this is a problem with all the P-30's or just certain years? In relation to the Master Cylinder where did you measure the 1 7/8 in distance?

[lonfu]

I've been wrenching for 45 years. Guess I didn't make it clear in my first post, I just replaced the front pads, rotors, master cylinder and bled all calipers. I've done hundreds of brake jobs. Brake pedal level started dropping on my way to Waldport oregon going up the grades, yes, up the grades! Had to go through several passes. But on the return trip I had to go over a 6k pass in Nevada. The brakes were not used going up the hill. The pedal went to the floor at the top of the grade, I had to pull over to let the brake lines and master cylinder cool off from the heat from the exhaust manifold. The master cylinder was Hot when I touched it. The steel brake lines that are 1 7/8 of an inch away from the manifold were to hot to touch. I don't think I can explain this any more clearly. The brake fluid was boiling in the master cylinder and brake lines. DOT 5.1 has the higher boiling point, but the exhaust manifolds can reach 1200 degrees. I bought the unit brand new in 98, no one else but myself has ever worked on this unit. The brake lines are in the original position, they were installed incorrectly or they forgot to install a heat shield.

No long runs up serious grades yet. I checked the flex lines, because boiling brake fluid can cause then them to delaminate internally, all flows are ok. Doesn't seem to be any damage to the abs unit. Remember the lines from the master cylinder lead directly to the abs unit. I may have gotten lucky in that I've caught it before any long term damage has been done.

[wolfe10]

OP, how old was the brake fluid when you drained it after this experience? What color was it?

New DOT III brake fluid boils around 500 degrees F. As it absorbs water (to protect the iron parts of your braking system) the boiling point lowers-- with most hydrated brake fluid having a boiling point of 286 degrees F.

BIG difference between 286 and 500 degrees!

Brake fluid should be changed every two years-- maybe every three if you never leave your desert location.

[enblethen]

Weak disc brakes on P-30s is nothing new. My '93 will heat the brakes up if I do not slow down at the top of the down grade, shift into a lower gear sometimes even low to descend.
Better brake fluid helps, and the pads should be severe duty ceramic(SX149 on mine) and not the standard pads.
Going down the grades, the engine exhaust should be cooling rather quickly. There will or should not normally be much heat transfer to the brake lines.
You could have other issues. If it has happened a couple times, the rubber brake lines could be collapsed on the inside restricting fluid flow and return.

[Class_A_DP_1]

This is on your 98 Itasca? How many test miles (passes) do you now have on your modification? Glad you are OK.