Volvo RWD 700 Forum

Hello,

I have an '85 745 with a front jumbo brake upgrade. Over the last month I've had a "weak" diagonal (LF, RR) and now the same diagonal was stuck this evening.

About 3 months ago I had the rear calipers off while doing a rear axle swap. I accidentally pressed the brake pedal down without a rotor between the rear pads. Both calipers pressed tightly together. I pulled them back. Upon getting the rotors back on, my RR caliper would not compress and the brake light was on. (brakes worked fine before this) I removed the caliper and confirmed the piston and dust boot were functioning fine with no rust. I thought that the deep compression of the master cylinder screwed it up so I replaced it. Still no luck, so I rebuilt the distribution block/brake light switch. Things worked OK for a while.

After about a month the LF/RR circuit started loosing brake power/pressure. It was intermittent at first (solid pedal, then soft at seemingly random times) but started getting worse. Then, I lost all power in that circuit and the brake light went on again. (rotors started to rust!) As my rebuild of the distribution block was suspect I purchased a new unit. Upon installation I still had no brake power in the LF/RR. I did notice that when I bled the brakes the LF/RR flow was very low compared to the RF/LR. I use a Motive pressure bleeder under 15psi max.

So, I got a new master cylinder to rule out an internal leak. In addition I blew out/cleaned the hard lines traveling from the MC to the distribution block to confirm there was no blockage. Pressure bleeding was good with equal flow in all calipers. Solid pedal feel with no brake light.

Two days later (today) the LF/RR calipers were both dragging enough to cause some shudder while driving. Hub temps on LF/RR were much hotter than RF/LR.

So, new MC, new distribution block. Calipers are both good and slide pins (front) are functioning correctly. I have IPD SS brake lines on all 4 corners. They are ~7 years old with ~30k miles. Only other thing to add is that this is a V8, so there is an exhaust manifold right next to the MC and distribution block. Though, In 7 years of driving I've never seen an issue. Brake fluid is Valvoline Synthetic Dot 3/4.

Any thoughts??? I'm going insane with this!

thanks,
simon

Well, I had 3 problems contributing to my brake woes.

1) My 3rd rebuilt MC was crap. The 4th one seems to be doing better
2) Because of all of the MC changes, my plunger pin was too far into the MC causing pressure, which is why my brakes locked
3) My hardline to the distribution block was not sealing very well, creating a small leak.

Hopefully everything will work better now!
-simon

Howdy Simon,

My guess, strictly a guess; debris in the LF/RR circuit that was blocking application pressure, is now holding residual pressure.

Install a pressure gauge that attaches to the brake bleeder to see if that is your problem. Here is one:

http://www.irvansmith.com/scart/quick-check-brake-pressure-gauge-p-2117.html

If any pressure is held on the LF/RR circuit after the pedal is released that would certainly cause the pads to drag against the rotors.

The good news is the debris, if that is indeed the problem, is now far enough along the line that pressure bleeding should push it out. Disconnect the calipers and use copious amounts of fluid to "wash" the debris down the line. Reverse bleed the calipers. Put the system back together and bleed by normal procedure.

The bad news is the shudder may be caused by the disk(s) and may be permanent until they are replaced. See this article:

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

If not debris in the circuit, I am stumped. Please do let us know what resolves the problem.
--
Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)

There may have been some debris in the system, though for various reasons, I've run about 2 gallons of brake fluid through the system in the last 3 months. Reverse flush didn't show anything but that doesn't mean I didn't have a chunk in there.

AFter replacing the MC, adjusting the pin that presses the MC, and replacing the hardline between the MC and the distribution block, things seem to be working OK. I went through a couple of heat cycles on the brakes and don't seem to have any long term effects. The rear rotors are getting changed soon with my 8.8 rear end swap. I have a spare pair of front rotors that I can swap if needed.

Thanks for the thoughts!
-simon

Hi Simon,


If your "Brake light" comes on aren't you losing brake fluid somewhere?
Look for leaks.

You said you replaced the master cylinder with a new unit. New as in
OEM Volvo new, or maybe "new" as in remanufactured?
How did the bench bleed go?
I'm on my third remanufactured M/C in a little over a year. You know you
have trouble when you can only depress the M/C piston half way without
adding air.

If you have a remanufactured M/C, I'd try another bench bleed and see if
it's free of air bubbles.

If the calipers are grabbing then I'd look for interference where the brake pads bear on the caliper castings, clean up any nasty spots with a file- Use a little bit of never seez where the pads ride against the calipers.
Check/lube the pins.

Bill

The brake light on the 700 series is set off by a pressure imbalance. You can get the light without loosing any fluid.

My calipers didn't have any problems. I pulled most of them apart and there was no issue. All had smooth bores with no corrosion. Slider pins were fine in the front.

The MC was a rebuilt unit... the 3rd. The 4th one seems to be doing better now, along with an adjustment of the pin that pushes it.

Seems to be working well.

thanks,
simon