pacerdonfl

I have two 91 Lex 400's. Been working on/around cars 50 years (I'm not a "certified" mechanic, but an engineer by trade).

This concerens my Lex with 150k miles on it - bought 2 yrs ago at 124k miles. Driving like a dream. Put new pads on front about 18 mos ago.

2 mos ago replaced rear brake pads. Simple job, no mess, all fine for about 3 weeks.
Then one day I was driving down a city road about 50-60 mph and all of a sudden it sounded and felt like I was running over rumble strips (the road was perfectly smooth) - very scary.
I pulled over to the side, looked under and all around, expecting to see a wheel falling off - all good, nothing loose.
I scratched my head, got back in the car and drove down the road and was all fine.
This happened randomly 3 more times in the next 10 days - all very scary - a heavy "thump, thump, thump" and vehicle jerking.....
I could only think my abs was going wacko, and not knowing anything about that, took out the fuse/breaker - not good. Put fuse/breaker back in.
Hasn't happened again since then - for about 4-5 weeks.
Now - the real mystery is, my brake reservoir has come to empty 3 times since this stuff began. That I discover when the brakes all of a sudden go almost to the floor.
So - three times I fill the reservoir to top, the brakes self bleed(?) in a day and are again working perfectly.

I CAN SEE NO VISIBLE (LEAKING?) BRAKE FLUID ANYWHERE NOWHERE!!!???
WHERE IS IT GOING?
THE BOOSTER (POWER BRAKE) IS WORKING FINE.

I have purchased a new master cylinder but not installed it yet.

Thanks for any thoughts.....


/Don

sha4000

iTrader: (1)

If the reservoir ran down to empty you have air in the system. YOU have to bleed the air out the system does not bleed itself. More importantly is where is the fluid going?

YoshiMan

Have you made a physical inspection of all the brakes to see if everything is where it should be?

I had a friend whose F-150 was making some odd on-and-off braking noises, and when we pulled everything apart, it turns out one of the rear calipers had it mounting bolts vibrate loose, and were totally missing. The caliper was just riding on top of the rotor and lucky for him, it didn;t move. Just glancing under the truck, it looked like everything was where it was supposed to be.

Superfast1

You must have a leak somewhere as it couldn't possibly deplete the fluid in the reservoir 3 times due to air as the system doesn't bleed nor recirculate the fluid by itself so check again. Do not replace the master cylinder if you don't see any sign of fluid leaking out between it & the power booster. Is your brake pedal hard & you had to apply more pressure to stop the car or does it feels spongy?

aptoslexus

Stop driving the car before you kill yourself or someone else until you figure out the leak. Take it to a pro who will go over the system. The fact that you topped the brake fluid off 3 times but you're still driving it is ridiculous.

spuds

Is it possible its leaking into the brake booster...anyone know?

Superfast1

On a 2nd thought, the fluid from the master cylinder could be leaking into the booster & caused the diaphragm inside the booster to fail. This could very well explain why you're losing fluid. To check if there's fluid inside the booster, you can remove the check valve from the booster then gently insert an oil dipstick into the hole where the check valve was (clean the dip stick first). If there's fluid on the dipstick then the master cylinder is leaking. To double-check & verify it, you can remove the 2 nuts where it's attached to the booster then pull the master cylinder out slightly w/o removing the brake lines then inspect for leak. Good luck.

jrk1956

IF the master cylinder is leaking into the booster, then the engine vacuum may be drawing the fluid into the intake manifold and the engine burning the fluid. That could be one reason you're not seeing any fluid leaking anywhere.

Gcuarino

off the subject. Buying a 92 with less than 100,000
great condition all paperwork steering, suspension and transmission replaced
1750$ is this a great deal?

spuds

Yes,if its all that its a steal IMO.Start a new post.

Superfast1

Yup. Exactly.

Gcuarino

I was going to flip it. If I did what would you suggest? But I am not!
I have been converted to the Lexus ls400 club!

spuds

Need to start a new thread,will get a ton of advice,people love those threads.

pacerdonfl

Thanks for checking in....it is now depleting the 4th reservoir....gotta be going into the booster...going to replace master cylinder, then I imagine the booster will go soon if it has enough of a leak to suck in brake fluid...???

pacerdonfl

Thanks - 2nd thought right on. The 3rd fill up of the reservoir is now depleting...brakes are tight and all working. Must be sucking it into the booster - nary a drop anywhere to be seen. I'm replacing the master cylinder today - I'm sure that soon the Booster will need replacing - if it is sucking in brake fluid then it's got a small leak in it's seal also....

Thank you for checking in.