Fading Brake Pressure, no fluid loss

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

S

Scooter

10 Year 15 Year
Joined
Mar 1, 2006
Threads
3
Messages
8
Location
San Jose, CA
Hi All,

95 FZJ80 with ~180,000 miles, brake system all original, brake fluid never changed/bled.

Last weekend I replaced the brake pads on all four corners, the installation was uneventful, I never opened the system (no air was introduced). After replacing the wheels I started the truck and pumped the brake pedal to set the calipers, the pressure never stabilized. I could pump the pedal to get pressure but as I applied steady pressure the pedal would sink to the final 2 inches (slowly and with moderate pressure). With the engine off the pedal is firm (does not fade). No fluid loss. I -properly- bled the system thoroughly with no improvement. I replaced the master cylinder (OEM), bled cylinder and system, no improvement. I bled with the key on, no improvement. There is no obvious soft line bulging.

Anyone have any ideas? Replies highly appreciated.
 
If you bleed the brakes the other thought is replace the brake fluid with new.
Water could of gotten in the lines at some point. Use DOT4 fluid higher boiling point.
Did you prime the new master cylinder before the install?
Is the booster line hooked up?
 
Offroads4x4 said:
If you bleed the brakes the other thought is replace the brake fluid with new.
Water could of gotten in the lines at some point. Use DOT4 fluid higher boiling point.
Did you prime the new master cylinder before the install?
Is the booster line hooked up?
Click to expand...
Thank you both for your replies, if you look carefully through the forum this is not a unique problem.

To answer your questions: Fluid completely replaced, I put nearly 2qts through it since I bled the brakes 4 times. Primed the master, booster lines checked. Checked behind dust seals in calipers, CDan quoted me a good price to replace all seven "soft" lines, I think this is my next move. If it works I’ll let you know. I am still interested if anyone has any more suggestions/success stories.

While I was down there I noticed the PHH is starting to leak, thank goodness there are several good write-ups on that repair.

I bought this truck when it had only 36K on it, it is the best vehicle I have ever owned.
 
yeah thats mt next move also is to replace the soft rubber lines...and also i have heard that using oem pads can make a big difference as well?
 
Scooter said:
Hi All,

95 FZJ80 with ~180,000 miles, brake system all original, brake fluid never changed/bled.

Last weekend I replaced the brake pads on all four corners, the installation was uneventful, I never opened the system (no air was introduced). After replacing the wheels I started the truck and pumped the brake pedal to set the calipers, the pressure never stabilized. I could pump the pedal to get pressure but as I applied steady pressure the pedal would sink to the final 2 inches (slowly and with moderate pressure). With the engine off the pedal is firm (does not fade). No fluid loss. I -properly- bled the system thoroughly with no improvement. I replaced the master cylinder (OEM), bled cylinder and system, no improvement. I bled with the key on, no improvement. There is no obvious soft line bulging.

Anyone have any ideas? Replies highly appreciated.
Click to expand...

If I understand you correctly that you push on the pedel and it sinks all the way to the floor, then the cause is most likely a bad master cylinder. The piston seal is no longer sealing. You'll notice this more with the engine running because the vacuum booster allows you to exert much more pressure, with less force against the master cylinder.
 
To save MC problems one should place a chunk of 2"x4" or similar product between the floor and brake pedal, this will prevent moving past the push rod seals, otherwise ensure that you stop the pedal before bottoming out.

Good luck.

Sometimes this problem cures itself as in an overnight rest...my guess is that the seals reseat themselves???
 
ok so ive already replaced my master cylinder cause my mechanic said that was the cause of the pedal going all the way to the floor...so i did that and it still goes to the floor and we also changed the brake fluid and bleed the mc properly ........so are yo usaying that toyota sold me a faulty part...... my next guess is the soft runner brake lines need to be changed andnew pads also?
 
yup, replace the brake fluid.


Brake fluid absorbs moisture, which eventually boils and there ya go, air in the system.

Easy fix

edit: oops, i skipped the post where you said you already did this.. lol
 
its stops ok i guess but i think once i get a few bucks together im going to toyota and get them to fix it
 
It probably just needs to be bled again. These 80s are extremely hard to bleed. Do a few searches and you'll see dozens of threads of people having to bleed, bleed, and bleed some more. Regular pumping of the brake to bleed is not adequate, especially if there's a lot of air in the system. They have to be power bled, either with a machine or pump. Even then, it could take a couple of cycles before its truly bled out properly.

The problem appears to be air that is trapped in certain parts of the system that can't be easy forced out via conventional bleeding techniques. One possible problem is air trapped in the ABS system and other is the LSVP bypass line is trapping air.

Another thing is the master cylinder needs to be adjusted when a new unit is installed. I think this is something unique to the 80 series as most indepedent mechanics won't do it. But Toyota techs should know about it. This adjustment affects things like pedal travel.
 
Thanks again to all who responded.

Air in the system doesn't make sense, it started before the system was "opened" and I am able to build pressure and THEN it fades.

After I install the soft lines (already on order) I will have to bleed it again. I'll let you know what I find out.

Thanks again.
 
the pedal adjustment is that somthing i can do or is it best to take it to a toyota stealership or someone who knows ALOT about toyotas ?
 
I had the same problem. Especially when the brakes got hot from going down hill. I just bled them again and again and found that the passenger front had the most air. So I bled it like 10 times. All the while putting in new fluid.

They came back to what I would call "spongy" normal.

Could a leaky booster hose from the throttle body add to the problem?
 
Still waiting for parts for brakes, I did get a chance to replace the pesky heater hose. Mine had a small rupture on the firewall side and was caught only because I was spending so much time under the car doing brakes. Arm length, width, and assortment of pliers have a great deal to do with how easy that project goes. For me, not so easy. I hate to question the engineers’ choices on that one but wow.
 
You must log in or register to reply here.

Share this page

Back
Top Bottom