Sinking brake pedal - Vintage Mustang Forums
 5Likes
Reply
 
Thread Tools
post #1 of 20 (permalink) Old 08-22-2019, 12:58 PM Thread Starter
Senior Member
 
qbui's Avatar
 
Join Date: Aug 2013
Location: Los Angeles, CA
Posts: 2,850
Sinking brake pedal

Hello everyone! So I've been experiencing this braking issue and just wanted to see what others think where the issue could lie. So the brake booster and master cylinder was replaced with a new Bendix booster & MC about 5 years back. When I step on the brakes, the pedal stops at the normal height but if I stay on the brakes, I can feel it sinking slowly. It's also evident when the Mustang is at a decline. I can feel it creeping down a little bit and have to press harder on the pedal. I've recent flushed the fluids to see if that fixes the issue but it didn't. Could it be a leak in the booster? I didn't noticed any leaks by the bleeding valves when I was bleeding the brakes. I don't hear any hissing sound like people mentioned if there's a leak in the booster. The valve from the engine vacuum to the booster? Anything else that it could be?
Appreciate any help! Thanks!

1969 Mustang Convertible
302, C4, P/S, P/B, A/C, FiTech EFI
qbui is offline  
Sponsored Links
Advertisement
 
post #2 of 20 (permalink) Old 08-22-2019, 01:06 PM
Senior Member
 
Hemikiller's Avatar
 
Join Date: Jun 2002
Location: Killingworth, CT
Posts: 8,877
Garage
If you have no obvious leaks, your master cylinder is bad.
Magnus, qbui, 2nd 66 and 1 others like this.

Nothing worthwhile is ever quick, cheap or easy, those that can't do, complain

71 Mach 1

HOLLEY TECH PAGES
FORD OE TACH HOOKUP FORD OE TACH HOOKUP w/MSD (pg 7)
Hemikiller is online now  
post #3 of 20 (permalink) Old 08-22-2019, 01:15 PM
Senior Member
 
jdub's Avatar
 
Join Date: Dec 2011
Location: Colorado
Posts: 4,643
Quote:
Originally Posted by Hemikiller View Post
If you have no obvious leaks, your master cylinder is bad.
I was going to say, it's really only two possibilities. Most people would notice a leak so it's likely a bad MC.

1965 Mustang
jdub is offline  
 
post #4 of 20 (permalink) Old 08-22-2019, 02:02 PM
Senior Member
 
Join Date: Oct 2015
Location: Anaheim
Posts: 1,189
Internal seal leak in the master . No fluid loss , fluid leaks by the seal on the piston.

'66 GT350H-3 time cover car - Car Craft July'77,Modified Mustangs and Fords Feb2011 ( w/article), Mustang Monthly June 2014. Bracket raced by me for the last 44 years. Yes it is a real one.
'68.5 R code GT fastback
'67 Ranchero 408W -427 glass hood
'78 F350 "Oleynik" car hauler
Ford Experimental parts collector
GT350HR is offline  
post #5 of 20 (permalink) Old 08-22-2019, 02:42 PM
Senior Member
 
Fitch's Avatar
 
Join Date: Aug 2019
Location: Fort Worth, Texas
Posts: 190
Also, get under the dash and peel back the rubber boot on the back of the master cylinder and check for fluid. Sometimes the boot will hold the fluid of a leaking master cylinder.

Cliff
1966 Blue Hardtop. 200 CID with 3 Speed Trans
Fitch is offline  
post #6 of 20 (permalink) Old 08-22-2019, 02:49 PM
Member
 
Join Date: Jul 2019
Location: WI
Posts: 34
Ditto on the master cylinder. To isolate certain parts an pieces on vehicle brake systems I have taken fittings that screw onto or into the suspected culprit. Sometimes you can get plugs that will seal off that circuit other times I've just welded the ends shut.
teteacher101 is offline  
post #7 of 20 (permalink) Old 08-22-2019, 03:17 PM Thread Starter
Senior Member
 
qbui's Avatar
 
Join Date: Aug 2013
Location: Los Angeles, CA
Posts: 2,850
OK, thanks for the info! I'll focus on the MC.

1969 Mustang Convertible
302, C4, P/S, P/B, A/C, FiTech EFI
qbui is offline  
post #8 of 20 (permalink) Old 08-22-2019, 08:08 PM Thread Starter
Senior Member
 
qbui's Avatar
 
Join Date: Aug 2013
Location: Los Angeles, CA
Posts: 2,850
So thinking about it, maybe at one time I may have bled the brakes and allow the fluid to run out couple years back. Could it be air in my MC and needs to be bench bled again?
Also about bench bleeding. Why can I set up the bleeding hoses while the MC is already installed and just pump the brake pedals? Why does it have to be done while out of the car?
Thanks!

1969 Mustang Convertible
302, C4, P/S, P/B, A/C, FiTech EFI
qbui is offline  
post #9 of 20 (permalink) Old 08-22-2019, 08:26 PM
Senior Member
 
Woodchuck's Avatar
 
Join Date: Mar 2001
Location: Peoples Republic of Vermont
Posts: 33,187
Send a message via Yahoo to Woodchuck
If you've got "that sinking feeling" in the brake pedal and you have no sign of an external leak, the fluid is probably passing the seals on the master cylinder and ending up in your booster. You can remove the nuts holding the master and just slide it forward a bit to check without having to un-do everything...

Bart

What, me worry?
- Alfred E. Neuman

Woodchuck is offline  
post #10 of 20 (permalink) Old 08-22-2019, 08:37 PM Thread Starter
Senior Member
 
qbui's Avatar
 
Join Date: Aug 2013
Location: Los Angeles, CA
Posts: 2,850
Quote:
Originally Posted by Woodchuck View Post
If you've got "that sinking feeling" in the brake pedal and you have no sign of an external leak, the fluid is probably passing the seals on the master cylinder and ending up in your booster. You can remove the nuts holding the master and just slide it forward a bit to check without having to un-do everything...
Thanks Bart! What am I looking for when I slide the MC out a bit? If there's fluid, the the MC, then it's bad and replace?

1969 Mustang Convertible
302, C4, P/S, P/B, A/C, FiTech EFI
qbui is offline  
post #11 of 20 (permalink) Old 08-22-2019, 08:49 PM
Senior Member
 
Woodchuck's Avatar
 
Join Date: Mar 2001
Location: Peoples Republic of Vermont
Posts: 33,187
Send a message via Yahoo to Woodchuck
Quote:
Originally Posted by qbui View Post
Thanks Bart! What am I looking for when I slide the MC out a bit? If there's fluid, the the MC, then it's bad and replace?
Yep. If it's damp you need a new master.

Bart

What, me worry?
- Alfred E. Neuman

Woodchuck is offline  
post #12 of 20 (permalink) Old 08-22-2019, 09:01 PM Thread Starter
Senior Member
 
qbui's Avatar
 
Join Date: Aug 2013
Location: Los Angeles, CA
Posts: 2,850
Quote:
Originally Posted by Woodchuck View Post
Yep. If it's damp you need a new master.
Thanks again Bart!
Woodchuck likes this.

1969 Mustang Convertible
302, C4, P/S, P/B, A/C, FiTech EFI
qbui is offline  
post #13 of 20 (permalink) Old 08-22-2019, 09:02 PM
Senior Member
 
Fitch's Avatar
 
Join Date: Aug 2019
Location: Fort Worth, Texas
Posts: 190
Quote:
Originally Posted by qbui View Post
So thinking about it, maybe at one time I may have bled the brakes and allow the fluid to run out couple years back. Could it be air in my MC and needs to be bench bled again?
Also about bench bleeding. Why can I set up the bleeding hoses while the MC is already installed and just pump the brake pedals? Why does it have to be done while out of the car?
Thanks!
No. Air in the system will make the pedal feel spongy but it will not "bleed" down.

Cliff
1966 Blue Hardtop. 200 CID with 3 Speed Trans
Fitch is offline  
post #14 of 20 (permalink) Old 08-22-2019, 09:42 PM Thread Starter
Senior Member
 
qbui's Avatar
 
Join Date: Aug 2013
Location: Los Angeles, CA
Posts: 2,850
Quote:
Originally Posted by Fitch View Post
No. Air in the system will make the pedal feel spongy but it will not "bleed" down.
Thanks for the info! Definitely not spongy.


So in the case that I need a new MC, why can't I bleed it after I installed it and use the bleeding hose kit instead of doing it on the bench which I don't have?

1969 Mustang Convertible
302, C4, P/S, P/B, A/C, FiTech EFI
qbui is offline  
post #15 of 20 (permalink) Old 08-22-2019, 10:21 PM
Senior Member
 
awhtx's Avatar
 
Join Date: Sep 2011
Location: The Hill Country of Central Texas
Posts: 9,176
Quote:
Originally Posted by qbui View Post
So in the case that I need a new MC, why can't I bleed it after I installed it and use the bleeding hose kit instead of doing it on the bench which I don't have?

You can do exactly as you stated it.
I don't even do that. Just put the MC in place, connect the brake lines and do the 2 man brake bleed. That gets rid of any air.
awhtx is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Vintage Mustang Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome