Magical Mystery Brake Problems
12-06-2011, 08:33 PM
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Magical Mystery Brake Problems
Chapter 1; History
I am the second owner and friend of the 1st owner of my 84 4Runner DLX. It has 250,000 miles and I have owned it since 2002. It has never been crashed.
In the past I have had problems with one of the rear wheels locking up at low speeds, especially right after the season's first rain. Bleeding the brakes solved the problem, but I had to do it each year at rainy season. Replacing the wheel cylinders did not fix it, nor did replacing drums, shoes or hardware. Brake lines showed no sign of blockage. No mechanic could figure it out. The solution was to bleed it annually after recurrence.
Chapter 2; Brake Failure And M/C Replacement No.1
One night exiting the freeway the brake pedal went to the floor. I pumped it and the pedal came up. Hooray, I didn't die. The problem is consistent with a typical failure of a piston seal within the master cylinder. No leaks were observed anywhere on the vehicle. The reservoir had no noticeable loss in volume. Oh, and I always use Synthetic Dot 3.
The brake master cylinder was replaced with a rebuilt unit. I bench bled the unit, and bolted it into place. Upon bleeding the brakes, I got constant air bubbles. That is, once the fluid that was already in the lines was flushed, only air bubble-infused fluid came through.
After bleeding it for a week, including using a vacuum bleeder, I took it to a mechanic who was also unable to bleed it of air. The conclusion was a bad rebuilt master cylinder.
Chapter 3; M/C Replacement No. 2
My plan was to return the rebuilt unit and purchase a new brake master cylinder, but while explaining this transaction to one auto parts clerk, the other clerk had already exchanged it for another rebuild unit. Apparently it was a PITA to un-exchange it and return it. Ok, fine.
Once home I again bench bled the (other rebuilt) master cylinder and bolted it into the 4Runner. This time no bubbles. Hallelujah.
With the engine off, the pedal height felt correct. It was also not spongy or rock hard. In other words, all seemed right. After starting the engine the vacuum kicked in, and the pedal dropped slightly as the brake booster did it's duty. Again, all was good.
However upon driving the 4Runner, the pedal dropped half to 2/3 of the way to the floor when I got to the stop sign.
The vehicle can stand still idling as long as I like and the pedal is fine. As soon as I start driving, the pedal will drop when braking. And it's not totally consistent either. Sometimes 100 feet is all it takes, other times 1/2 mile or more. Sometimes the pedal drops 1/4 of the way, sometimes 3/4, sometimes in between.
Chapter 4; Replacing Unrelated Items
After consulting a mechanic, he told me that it must be my parking brake cable which was out of adjustment. True, it was. However it had nothing to do with the hydraulic system. I adjusted it anyway since it needed doing, and to rule it out.
Next he told me that I needed new brake shoes. I did not as they were practically new. Rather than argue, I replaced perfectly good shoes. Ruled out. No improvement. And yes, the star wheels are properly adjusted.
Chapter 5; The Mystery Deepens As The Wallet Thins
After consulting manuals, Google, and several opinions (including the dealership) the list was narrowed to three likely culprits: 1) Another bad master cylinder (unlikely), 2) Bad brake booster, and 3) Bad Load Sensing Proportioning Valve (LSPV). The booster was replaced first per the recommendation of the dealership, but to no avail. The LSPV was next, and it was pricey as genuine Toyota (Mfd. by Aisin, dealer only) was the only option. Again no improvement. Finally I ordered a NEW master cylinder, manufactured by the OEM Aisin.
After replacing all of these, the problem remained exactly the same: Pedal height drops only when the vehicle is moving, and is still consistently inconsistent.
Chapter 6; Brake Hoses, Wheel Cylinders And Calipers
The next things to be replaced were all 3 brake hoses, Raybestos brand. No dice, Spicoli.
The decision was then made to replace the remaining components starting with the rear wheel cylinders. Remember that they were replaced a few years ago erroneously. They were replaced again with new Raybestos brand cylinders. At this point I have hauled more bottles of Dot 3 from the store than a Kentucky shine runner hauls rotgut. I mean, I'm bleeding my brakes in my sleep. The problem remains.
Next to go are the calipers. As it turns out, no NEW replacements of the exact original type were available. Only rebuilt. So, I opted to rebuild them myself as the rebuild kits were around $5 each. It was actually surprisingly easy as well. The chrome on the pistons was without pitting, and had only minor surface abrasions. One cylinder cavity in the driver's side caliper had a small bit of rust, but behind the O-ring. The rust was scraped out leaving a shallow pit. It should have no effect on operation. No leaks are observed. The calipers were then placed back in operation only to exhibit the same symptoms.
Chapter 7; The Saga Continues
To recap, I have:
I am the second owner and friend of the 1st owner of my 84 4Runner DLX. It has 250,000 miles and I have owned it since 2002. It has never been crashed.
In the past I have had problems with one of the rear wheels locking up at low speeds, especially right after the season's first rain. Bleeding the brakes solved the problem, but I had to do it each year at rainy season. Replacing the wheel cylinders did not fix it, nor did replacing drums, shoes or hardware. Brake lines showed no sign of blockage. No mechanic could figure it out. The solution was to bleed it annually after recurrence.
Chapter 2; Brake Failure And M/C Replacement No.1
One night exiting the freeway the brake pedal went to the floor. I pumped it and the pedal came up. Hooray, I didn't die. The problem is consistent with a typical failure of a piston seal within the master cylinder. No leaks were observed anywhere on the vehicle. The reservoir had no noticeable loss in volume. Oh, and I always use Synthetic Dot 3.
The brake master cylinder was replaced with a rebuilt unit. I bench bled the unit, and bolted it into place. Upon bleeding the brakes, I got constant air bubbles. That is, once the fluid that was already in the lines was flushed, only air bubble-infused fluid came through.
After bleeding it for a week, including using a vacuum bleeder, I took it to a mechanic who was also unable to bleed it of air. The conclusion was a bad rebuilt master cylinder.
Chapter 3; M/C Replacement No. 2
My plan was to return the rebuilt unit and purchase a new brake master cylinder, but while explaining this transaction to one auto parts clerk, the other clerk had already exchanged it for another rebuild unit. Apparently it was a PITA to un-exchange it and return it. Ok, fine.
Once home I again bench bled the (other rebuilt) master cylinder and bolted it into the 4Runner. This time no bubbles. Hallelujah.
With the engine off, the pedal height felt correct. It was also not spongy or rock hard. In other words, all seemed right. After starting the engine the vacuum kicked in, and the pedal dropped slightly as the brake booster did it's duty. Again, all was good.
However upon driving the 4Runner, the pedal dropped half to 2/3 of the way to the floor when I got to the stop sign.
The vehicle can stand still idling as long as I like and the pedal is fine. As soon as I start driving, the pedal will drop when braking. And it's not totally consistent either. Sometimes 100 feet is all it takes, other times 1/2 mile or more. Sometimes the pedal drops 1/4 of the way, sometimes 3/4, sometimes in between.
Chapter 4; Replacing Unrelated Items
After consulting a mechanic, he told me that it must be my parking brake cable which was out of adjustment. True, it was. However it had nothing to do with the hydraulic system. I adjusted it anyway since it needed doing, and to rule it out.
Next he told me that I needed new brake shoes. I did not as they were practically new. Rather than argue, I replaced perfectly good shoes. Ruled out. No improvement. And yes, the star wheels are properly adjusted.
Chapter 5; The Mystery Deepens As The Wallet Thins
After consulting manuals, Google, and several opinions (including the dealership) the list was narrowed to three likely culprits: 1) Another bad master cylinder (unlikely), 2) Bad brake booster, and 3) Bad Load Sensing Proportioning Valve (LSPV). The booster was replaced first per the recommendation of the dealership, but to no avail. The LSPV was next, and it was pricey as genuine Toyota (Mfd. by Aisin, dealer only) was the only option. Again no improvement. Finally I ordered a NEW master cylinder, manufactured by the OEM Aisin.
After replacing all of these, the problem remained exactly the same: Pedal height drops only when the vehicle is moving, and is still consistently inconsistent.
Chapter 6; Brake Hoses, Wheel Cylinders And Calipers
The next things to be replaced were all 3 brake hoses, Raybestos brand. No dice, Spicoli.
The decision was then made to replace the remaining components starting with the rear wheel cylinders. Remember that they were replaced a few years ago erroneously. They were replaced again with new Raybestos brand cylinders. At this point I have hauled more bottles of Dot 3 from the store than a Kentucky shine runner hauls rotgut. I mean, I'm bleeding my brakes in my sleep. The problem remains.
Next to go are the calipers. As it turns out, no NEW replacements of the exact original type were available. Only rebuilt. So, I opted to rebuild them myself as the rebuild kits were around $5 each. It was actually surprisingly easy as well. The chrome on the pistons was without pitting, and had only minor surface abrasions. One cylinder cavity in the driver's side caliper had a small bit of rust, but behind the O-ring. The rust was scraped out leaving a shallow pit. It should have no effect on operation. No leaks are observed. The calipers were then placed back in operation only to exhibit the same symptoms.
Chapter 7; The Saga Continues
To recap, I have:
- Replaced the master cylinder, three times
- Adjusted the emergency brake cable
- Replaced perfectly good brake shoes
- Properly adjusted the star wheel
- Replaced the brake booster
- Replaced the LSPV
- Replaced all 3 brake hoses
- Replaced both rear wheel cylinders
- Rebuilt both front calipers
12-07-2011, 09:29 AM
#2
Uhhh...... yeah.... No, not really....
But I will certainly contemplate this...
How about booster check valve and/or vacuum source? That may affect booster and pedal amplification. And your certain that there is absolutley no issues with that pitting and such in the calipers? Because that can affect the smooth movement of the piston. I am not sure of where you're describing the pits are, thats why I am asking.
But I will certainly contemplate this...
How about booster check valve and/or vacuum source? That may affect booster and pedal amplification. And your certain that there is absolutley no issues with that pitting and such in the calipers? Because that can affect the smooth movement of the piston. I am not sure of where you're describing the pits are, thats why I am asking.
12-07-2011, 03:13 PM
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How about booster check valve and/or vacuum source? That may affect booster and pedal amplification. And your certain that there is absolutley no issues with that pitting and such in the calipers? Because that can affect the smooth movement of the piston. I am not sure of where you're describing the pits are, thats why I am asking.
(hold music)
Ok, I'm back. Check valve checks out and can be checked off the checklist.
I certainly can test the vacuum source. The problem does not occur when the vehicle is stationary. Even if I rev the engine. I tried idle, 1500 rpm, 2000, 2500, 3000, and more. The problem only seems to occur during actual movement of the vehicle. Which is why suspecting the caliper makes sense.
For readers unfamiliar with the calipers, it is basically an iron casting with 4 cup shaped cavities, 2 on each side of the rotor. Within each "cup" is a machined recess near the opening, which accepts a rubber O ring. The bore is otherwise smooth. The piston moves outward in that "cup" when the pedal is pressed. Fluid is sealed in by the O ring.
The pit in question is located on that smooth bore further in from the O ring. It is small, and has left no marks on the piston (which would suggest interference). I also scraped a small bubble of rust out of it. In theory, the rust "bubble" would have been scraped smooth and/or left a mark on the piston if it had interfered.
But yes, replacement calipers are next on the shopping list. I will also test the vacuum.
12-07-2011, 03:18 PM
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12-08-2011, 11:40 AM
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12-08-2011, 11:45 AM
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10-03-2012, 10:04 AM
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I wouldn't place the blaim on the calipers. They act the same whether the truck is on or off. Sounds like the vacuum from the engine is engaging the brake booster but idk. Have you checked the pedal pushrod adjustment?
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