Myst420

Hey everyone,

My boss's 99 RX300 recently started having some brake issues.

I'll start with the pads and rotors were all changed about a year ago (by me) and everything has been fine up until now. The RX has lost almost all of its braking power. Sometimes the pedal will drop to the floor (giving the feel of air in the line) but if you go to "pump" the brakes, the pedal becomes VERY stiff and takes a lot of force to press it. He told me he topped the fluid off last week and said it was near the 'min' level. (of course this happens while I'm on vacation and wasn't able to see exactly how low it was myself).

I've already told him to only drive it if its an emergency, so he's driving his son's car in the meantime.

Could this be the master cylinder? Brake booster? How should I go about diagnosing?

Thanks,
Mike

salimshah

Where is the fluid going?

Once the level dropped air could be in the system too.

I would look for leaks and fix that and then bleed the brakes.

Salim

Myst420

I was able to look at it for a few minutes this am. I check for leaks and didn't see any. I'm not 100% convinced that there is a leak. I'm thinking the fluid was low because of the pads being worn, and he got paranoid. He said the level was above the min line, so I don't think air would have gotten in. While sitting at idle, the pedal would get HARD, and then sponge its way all the way to the floor, so I'm leaning more towards master cylinder, but haven't had a chance to really go over everything yet.

JAB

One possible explanation is a faulty OCV (oil control valve) which somehow affects the vacuum available to the brake booster. However, my recommendation is that you/he take the RX to a competent mechanic; brakes are nothing to mess around with.

goosemac

Does the car shake on idle when the brakes are becoming stiff? I had this problem and fixed it two weeks ago by changing the oil control valves

Found the DIY here which was extremely helpful

https://www.clublexus.com/forums/rx-...ement-diy.html

Myst420

Hey guys, sorry for the delay in response....The car has been parked for over a week now.. I looked at it yesterday, and the only codes its throwing are P0171 and P0446. Could the OCV still be the issue, even if its not throwing a code for them?

The RX also stalled on me a couple times when I hit the brake pedal while the car was idling (if this gives any insight to what the issue might be)

goosemac

My RX never stalled on me, and I drove it for 2 months with the brake issue, so I'm not sure.

My car threw out different codes when disconnecting the negative battery cable. Try that, see if the codes change.

driver1988

start with the cheapest and easiest fix. If no external leak, bleed the brake lines for air. If no air, it could be BMC. When you bleed your brakes, test each caliper for pressure. If any single caliper does not hold pressure, then there is internal leaking in the BMC.

Now, regarding the stiff brake pedal, test it out by pumping brakes with engine off. Pump multiple times until firm. Insert key to light up dashboard. Pedal should sink, meaning booster is okay.