Brake pedal sinks to floor

AJBASS

MEMBER
2000 SUBARU FORESTER
2.5L
4 CYL
4WD
AUTOMATIC
226,000 MILES

We just had the a new brake power boost unit, new rear drums, brake shoes, and one wheel cylinder replaced (that was leaking) by our local mechanic. Now, when we start the car with our foot on the brake like we always do, the pedal immediately sinks down to the floor. It also sinks down to the floor after coming to a full stop. Otherwise, the brakes themselves feel fine during routine braking when driving. At first, the mechanic used aftermarket brake shoes, and we were experiencing a clunking noise along with a pulsation as I braked from around 30 MPH to a stop, and it was felt in the pedal, as well as the entire motion of the car. I thought it felt like a bad/out-of-round drum, or perhaps a drum with a high spot in it. (BTW, I actually went to Lincoln Technical Institute years ago and almost became an Auto Technician, but went into another field instead, so you can talk tech talk with me.) So, I'm now thinking perhaps the new wheel cylinder he installed might be the culprit? Or, the master cylinder has failed, but I don't personally have time to check it. But the fluid is full in the reservoir, and I don't see anything leaking from it, or from any wheel, or anywhere else. And, the mechanic is basically scratching his head saying, "I don't know what it could be?" What do you think? Thank you in advance for your help! Much appreciated!

5 Replies

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

Welcome to 2CarPros.

Honestly, my first suspect is the master cylinder. The plungers in it use the same area for years. Then, work is done, the system is bled, and the plungers now go to the end of their travel into dirt, corrosion, or anything that has built up. As a result, the plungers are damaged and allow fluid to bypass and the pedal goes to the floor. This past Saturday, I was called in on a similar issue. A brake line ruptured on an S-10. Line was replaced, but they couldn't get proper bleeding and the pedal went to the floor. I explained the same thing, so the tech replaced the MC and after that, the brakes were fine.

If there are no leaks, this is my primary suspect.

Take a look at these links:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

___________________________________________________

Here are the directions for replacing the MC specific to your vehicle. The attached pics correlate with the directions.

__________________________________________________

PROCEDURES
A: REMOVAL
1. Thoroughly drain brake fluid from reservoir tank.
2. Disconnect fluid level indicator harness connector.
3. Remove brake pipes from master cylinder.
4. Remove master cylinder mounting nuts, and take out master cylinder from brake booster.

CAUTION: Be extremely careful not to spill brake fluid. Brake fluid spilt on the vehicle body will harm the painted surface; wipe it off quickly if spilt.

B: DISASSEMBLY
1. PRECAUTIONS FOR DISASSEMBLING
1. Remove mud and dirt from the surface of brake master cylinder.
2. Prepare tools necessary for disassembly operation, and arrange them neatly on work bench.
3. Clean work bench.

2. DISASSEMBLING PROCEDURE
1. Remove reserve tank.

pic 1

2. Remove cylinder pin. (only vehicle equipped with ABS)
3. Pry up the pawl and remove the piston retainer.

pic 2

NOTE: Piston may jump out from master cylinder.

4. Extract primary piston assembly and secondary piston assembly.

CAUTION:
- Do not disassemble the piston assembly; otherwise, the spring set value may be changed.
- Use brake fluid or methanol to wash inside wall of cylinder, pistons and piston cups. Be careful not to damage parts when washing. If methanol is used for washing, do not dip rubber parts, such as piston cups, in it for more than 30 seconds; otherwise, they may become swelled.

C: INSPECTION
If any damage, deformation, wear, swelling, rust, and other faults are found on the primary piston assembly, secondary piston assembly, supply valve stopper, or gasket, replace the faulty part.

CAUTION:
- The primary and secondary pistons must be replaced as complete assemblies.
- The service limit of the clearance between each piston and the master cylinder inner dia. is 0.11 mm (0.0043 inch).
- When handling parts, be extremely careful not to damage or scratch the parts, or let any foreign matter get on them.

D: ASSEMBLY
1. PRECAUTIONS FOR ASSEMBLING
1. When assembling, be sure to use recommended brake fluid.
2. Ensure that the inside wall of cylinder, pistons, and piston cups are free from dirt when assembling.
3. Be extremely careful not to damage, scratch, or dent cylinder inside wall, pistons, and piston cups.
4. Do not drop parts. Never attempt to use any part that has been dropped accidentally.

2. ASSEMBLING OPERATION
1. Assembling piston assembly:
Apply recommended brake fluid to inside wall of cylinder, and to outer surface of piston assembly, and install piston assemblies carefully into cylinder.
2. Assembling cylinder pin:

pic 3

3. Press the pawl and install the piston retainer into the master cylinder.

E: INSTALLATION
To install the master cylinder to the body, reverse the sequence of removal procedure.
Tightening torque:
Master cylinder mounting nut 14 ± 4 Nm (1.4 ± 0.4 kg-m, 10.1 ± 2.9 ft. lbs.)
Piping flare nut 15 +3/-2 Nm (1.5 +0.3/-0.2 kg-m, 10.8 +2.2/-1.4 ft. lbs.)

CAUTION: Be sure to use recommended brake fluid.
________________________________________

Let me know if this helps or if you have other questions.

Take care,
Joe

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Wednesday, May 15th, 2019 AT 8:41 PM

AJBASS

MEMBER
3 POSTS

Thank you very much for all of that information, Joe! Well, our mechanic replaced the master cylinder, power bled the system, and the pedal still went to the floor. I stood by and watched him and chatted with him the entire time. So, since I almost became a tech myself years ago, I too know more than a thing or two about cars. So, just as I was about to make the same suggestion, he said, "Hmm. Maybe I should also bleed the ABS lines." I said, "That sounds like a great idea." Since he was the only person at the shop, I helped him by pumping then holding down the pedal as he cracked each line open. He gets to the third line, and he goes, "Ahhh!". Well what do know? Air bubbles! He felt bad and didn't charge me for the new master cylinder! Brakes work perfectly now. I suspected air in the system right from the beginning, right after the original job was done. I just assumed that when he told me that he bled the system, that he must've included the ABS part of it as well, so it never occurred to me to even ask. It had all the symptoms of air in the lines, because the car was stopping perfectly fine, but the pedal would sink to the floor only after I came to a complete stop, but the car remained fully at a stop. Also, with the car off, the pedal was perfectly firm. So, there ya have it! Lesson learned. Always bleed the ABS system as well, and be sure the tech does it! Thank you again, Joe! My best!

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Sunday, May 19th, 2019 AT 7:36 PM

JACOBANDNICKOLAS

MECHANIC
108,289 POSTS

Welcome back:

First, I'm glad it is finally fixed for you. But, this has to be frustrating at the same time. LOL Regardless, like you said, live and learn. By the way, thanks for letting me know.

You take care of yourself and feel free to stop back anytime you need something.

Joe

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Sunday, May 19th, 2019 AT 8:24 PM

AJBASS