Belrix
New member
Hi all, first posting.
It's a 1990 4Runner with 240,000 miles, auto-trans, v6 3.0 engine. It was recently, heavily rebuilt (engine, trans, suspension), by the previous owner and then sold to me about a week ago.
This Saturday, I was driving up the mountains for the first time since I've gotten the car. The engine is small so I spent a lot of time downshifted and climbing slowly up the mountainside (to 9300 feet alt).
I was following another vehicle and when we got to the top, the vehicle in front hit their brakes for some reason. I went to hit mine and "Whoops!" the pedal slid nearly effortlessly to the floor. I swung right, along-side the car and kept pressing the brakes. When the pedal was nearly at the floor, the brakes grabbed, I slowed quickly, and I pulled back into my lane.
When I reached the top and the engine dropped back to a more normal RPM, the brakes returned. A couple times this happened. When the engine was laboring for a longish period of time, the brakes would go away then return fairly quickly as the average RPM dropped back to normal. They seem to come back pretty quickly - in a mile or two.
The 4Runner has a history of a slightly squishy brake pedal but this seems ridiculous. The brakes work find down here in the "flats". No problems at all.
I asked a mechanic friend at the mountain top (group camping weekend) and he said that he thought I was losing my master cylinder. If it heats up, as the engine is working harder, then the braking strength drops. When I returned to a more normal RPM, the master cylinder would cool and brakes would return. He said if it was a vacuum leak, then the pedal would get hard, not soft.
The brake fluid is full, even slightly over the "Full" line but not badly so. Also, FWIW, the engine runs a little hotter than normal because the previous owner replaced the overhead cams, breather, & exhaust manifold with "performance" parts.
I came down the mountain two days later with no problems. This, of course, is the easy direction. I tend to downshift to slow as a practice as to not overwork the brakes. It's a pretty standard trick here in Colorado.
So - any other theories before I drop $130 on a new master cylinder?
My working theory, by the way, is that the brake fluid is old and it's boiling. Maybe replacing the fluid with some high-temp stuff is the right path.
It's a 1990 4Runner with 240,000 miles, auto-trans, v6 3.0 engine. It was recently, heavily rebuilt (engine, trans, suspension), by the previous owner and then sold to me about a week ago.
This Saturday, I was driving up the mountains for the first time since I've gotten the car. The engine is small so I spent a lot of time downshifted and climbing slowly up the mountainside (to 9300 feet alt).
I was following another vehicle and when we got to the top, the vehicle in front hit their brakes for some reason. I went to hit mine and "Whoops!" the pedal slid nearly effortlessly to the floor. I swung right, along-side the car and kept pressing the brakes. When the pedal was nearly at the floor, the brakes grabbed, I slowed quickly, and I pulled back into my lane.
When I reached the top and the engine dropped back to a more normal RPM, the brakes returned. A couple times this happened. When the engine was laboring for a longish period of time, the brakes would go away then return fairly quickly as the average RPM dropped back to normal. They seem to come back pretty quickly - in a mile or two.
The 4Runner has a history of a slightly squishy brake pedal but this seems ridiculous. The brakes work find down here in the "flats". No problems at all.
I asked a mechanic friend at the mountain top (group camping weekend) and he said that he thought I was losing my master cylinder. If it heats up, as the engine is working harder, then the braking strength drops. When I returned to a more normal RPM, the master cylinder would cool and brakes would return. He said if it was a vacuum leak, then the pedal would get hard, not soft.
The brake fluid is full, even slightly over the "Full" line but not badly so. Also, FWIW, the engine runs a little hotter than normal because the previous owner replaced the overhead cams, breather, & exhaust manifold with "performance" parts.
I came down the mountain two days later with no problems. This, of course, is the easy direction. I tend to downshift to slow as a practice as to not overwork the brakes. It's a pretty standard trick here in Colorado.
So - any other theories before I drop $130 on a new master cylinder?
My working theory, by the way, is that the brake fluid is old and it's boiling. Maybe replacing the fluid with some high-temp stuff is the right path.
