Whining brake pedal

Question

When I turned the ignition to on and put my foot on the brake pedal yesterday it was whining like a little motor was running. We drove around for a few miles. I noticed the brakes got spongier but not consistently.
I pulled the cap off the 3 month old master cylinder and noticed the rubber bulbs popped out. I pushed them back in and added some brake fluid and searched for a leak we must have. There’s a small one at one of the junctions of the new lines they replaced 3 months ago. Put the lid back on and drove an hour no problems, no noise.
Parked overnight and now the noise is back this morning.
Pulled the cap off again and noticed the rubber inside that has the bulbs on it is super loose and one bulb was partially popped out again.
Now the noise happens when I press the brake pedal even when the key is off.

1990 Bounder 40z Ford fuel injection v8 Oshkosh chassis

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enblethen · Answer

ABS system would be first guess.Then the auto park system if equipped.

kimberlysmoore · Answer

enblethen wrote:ABS system would be first guess.Then the auto park system if equipped.Turned out the minor leak is NOT brake fluid as I thought. It’s power steering fluid (pink).Bleeding the brakes tomorrow. The sponginess calls for it. Found a 2nd master cylinder in the rear supplying the rear main and tag axle. Might need to be replacedI’m pretty sure a whirring  sound was present prior to the front master cylinder going out about 3 or 4 months ago. Have to start somewhere. So so sorry for posting in the wrong place. Drop down menus and sub menus are not working on my phone browser since the last OS update.

77rollalong · Answer

the winning noise could be the backup pump for the hydro boost, may be an idea to check all the brake lines for any signs of leaks if the master cyl is dropping in fluid level that quickly. They will drop over time as the disk brake pads wear, but not in a few days it should not..

hank_mi · Answer

77rollalong wrote:the winning noise could be the backup pump for the hydro boost, may be an idea to check all the brake lines for any signs of leaks if the master cyl is dropping in fluid level that quickly. They will drop over time as the disk brake pads wear, but not in a few days it should not..I'm sure it's the hydro boost, ours works the same. Anytime you press the brake with the engine not running and providing vacuum to the master cylinder it will run. It's a safety thing. If the engine quits while your moving you'll need some help stopping that rig. Disk brake take a lot of pressure to work. Something you probably can't do without assistance. Don't worry about the rubber bulbs popping out as long as the fluid level isn't dropping. Every time you press on the brakes the fluid level will drop some, sucking the bulbs down. Release the brakes and the fluid level goes back up but may not push the bulbs up.

wa8yxm · Answer

Ok many notorhoes have what is called a HYDROBOOST brake system. In this system the POWER STEERING pump provides assist to the brakes.

IF the PS pump is not pumping, be it low on fluid or otherwise not doing it's job, or even if at full idle it is not pumping fast enough a FLOW SWITCH closes (normally closed, opens if flow) and and electric pump "Assists" your brakes.

I would suggest a full brake system inspection by a professional given the other symptoms you cited.

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Whining brake pedal

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