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Odd brake behaviour

MichaelJC

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May 13, 2021
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new_zealand
1994 80 series. Never had this problem before. Owned it 20 years. Never run out of fluid and no leaks.

Start out in the morning and brakes feel normal. After a long straight, suddenly spongy (almost to the floor, very scary). Pumps up to firm again.

I did a basic bleed. No change. Have a new master cylinder kit. Will try this but still would like to know why this is happening? It is very strange. Its a new behaviour.

Thanks

MC
 
Could easily be Master cylinder.. Change it then go from there if its no better..... Check all 4 wheels are Braking and nothing is sticking, Just eliminate problems with sticking/seized/knackered/ leaking, brake callipers/pistons etc etc..... But id bet a NZ Dollar its the Master cylinder... Good luck....PS is it the ABS Model ?
 
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Thanks guys. No ABS. No caliper sticking. Will fit MC kit. It just mystifies me that a seal could work fine then get sloppy a few KM down the track. Usually a faulty seal is spongy at the very first push of the pedal. Its a very dangerous scenario.
 
Also worth checking for any excessive free play in a wheel bearing. I've seen it before and rules it out (pushes the caliper pistons in)
 
Happened on my 80 towing last year. Quite disconcerting, it was a seized caliper causing brake fluid to boil. It looks like you have ruled out that possibility.
 
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Hi mate,

I’m having the save problem! I have have a 1993 with 104400 miles. My problem is happening after a long descend from 3700 meters in altitudes the pedal goes to the floor at 2000 meters. With a steep descent and a twisty road, it is a good feeling! I changes the brake fluid a few weeks ago, and I thought I solved the problem until my last descent! Next week end I will service my calipers and work my way up to the master cylinder and the booster which represent the end of the brake line. I find the seized piston diagnostic interesting and logical. I will keep you posted with my progress hoping it will help you as well.
 
Do the 80 series have some form of load valve on the rear brakes? Just wondering as a cousin of mine had a Patrol he fitted with extra air booster springs. That changed the axle to body height and meant that on long descents the front brakes were doing all the work.
 
Hi mate,

My problem is happening after a long descend from 3700 meters in altitudes the pedal goes to the floor at 2000 meters. With a steep descent and a twisty road, it is a good feeling!
I had this when off roading across The Pyrenees a few years back.
The only fix at that time was steep descents (on or off road) were done in 4 low.
Do the 80 series have some form of load valve on the rear brakes?
There is a load sensing proportioning valve on the back axle, just above the rear diff.
It should be adjusted if you’ve fitted a suspension lift. Or at least checked to make sure it’s operating OK.
Not sure this is the issue with regards to the original post though
 
Thanks for your time in relating your experience. I don’t have lifted suspension, but your idea is not bad and sure worth investigating. It makes sense to explore this option, since my rear pads are almost new and the front one are worn out. I bought the truck last July and unfortunately I don’t have the maintenance history. By eliminating each probable cause, I’m certain I’ll find the cause and everybody will learn out of it. Again thanks for your advise, I’ll keep you posted.
cheers
 
Even the smallest fluid leak will show if there are any, air leaks on the other hand are harder to find, I would be inclined to look towards the master cylinder first, incidentally if the master cylinder is worn bleeding the brakes often makes the problem worse.


Regards

Dave
 
Here’s an update on my brakes. I changed the seals on my front calipers last weekend. The pistons were dirty but not seized or scratched. I cleaned up everything and put everything back in place. Bled the lines and wished for the best. I noticed that my pads are at the wear mark and poor quality. So changing them is my next weekend job. I gave it a good road test on Sunday (200 km on high mountain roads with long 10+ km descents). So far so good! But definitely thin pads are bringing more heat into the caliper’s brake fluid. I also bled the rear proportioning valve, but nothing abnormal to declare. Keep you guys posted. Cheers!
 
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