Brake pedal sinks to floor engine on

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Cdog_96

Junior Member
Joined
Jan 15, 2022
Posts
17
Reaction score
11
Location
Aiken
First Name
Clay
Truck Year
1983
Truck Model
K10
Engine Size
454
I am stumped, I have replaced my brakes all the way around, new MC and brake booster, bled the lines several times and my brake pedal goes all the way to the floor with the engine on. With the engine off the pedal feels good and doesn't sink, with the vacuum booster plugged off pedal feels good also. It seems the booster is working due to it over assisting the pedal with the engine on. Master cylinder is a new ac Delco. No fluid leaks anywhere, I don't really know where to start and I don't wanna replace new parts unless I have to . Any help appreciated thank you
 

squaredeal91

Full Access Member
Joined
Mar 10, 2023
Posts
1,787
Reaction score
3,141
Location
Cave junction Oregon
First Name
Greg bush
Truck Year
1991 SB
Truck Model
K30
Engine Size
5.9 Cummins 12 valve
I know you said new master cylinder but it sounds just like a bad mater. Did you bench bleed it good before installing?
 

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
4,060
Reaction score
5,896
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
Plug all the brake ports at the master,if the problem goes away replace just your front lines. Did the problem stay away? Replace your rear lines,did the problem come back? Now we've narrowed out a lot. Use this strategy in which ever combination you need too,to narrow down the problem.
 

Cdog_96

Junior Member
Joined
Jan 15, 2022
Posts
17
Reaction score
11
Location
Aiken
First Name
Clay
Truck Year
1983
Truck Model
K10
Engine Size
454
I know you said new master cylinder but it sounds just like a bad mater. Did you bench bleed it good before installing?
Yes I spent a good hour on it, but it has been a while so maybe a possibility
 

Cdog_96

Junior Member
Joined
Jan 15, 2022
Posts
17
Reaction score
11
Location
Aiken
First Name
Clay
Truck Year
1983
Truck Model
K10
Engine Size
454
Plug all the brake ports at the master,if the problem goes away replace just your front lines. Did the problem stay away? Replace your rear lines,did the problem come back? Now we've narrowed out a lot. Use this strategy in which ever combination you need too,to narrow down the problem.
What do you mean plug the brake ports ? You mean where the brake lines go in ? How will I do any brake operation I think I am misunderstanding
 

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
4,060
Reaction score
5,896
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
Where the brake lines screw into the master get plugs and plug it does your pedal still sink. Ta Da the problem is at the master. Things are fine hook up the fronts,does the problem come back? Ta Da the problems in the front No ? Hook up the rears,Does the problem come back? Ta Da the problems in the rear.
 
Last edited:

Cdog_96

Junior Member
Joined
Jan 15, 2022
Posts
17
Reaction score
11
Location
Aiken
First Name
Clay
Truck Year
1983
Truck Model
K10
Engine Size
454
Where the brake lines screw into the master get plus and plug it does your pedal still sink. Ta Da the problem is at the master. Things are fine hook up the fronts,does the problem come back? Ta Da the problems in the front No ? Hook up the rears,Does the problem come back? Ta Da the problems in the rear.
Ahhh I gotcha, do this with engine running correct ?
 

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
4,060
Reaction score
5,896
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
Ahhh I gotcha, do this with engine running correct ?
Do it j7st like you are now, engine off,then engine running. Start reconnecting things until your symptoms come back.
 

squaredeal91

Full Access Member
Joined
Mar 10, 2023
Posts
1,787
Reaction score
3,141
Location
Cave junction Oregon
First Name
Greg bush
Truck Year
1991 SB
Truck Model
K30
Engine Size
5.9 Cummins 12 valve
Those hose pinchers are nifty! I drove around for a week or 2 with vice grips on my front caliper hose until I could afford a new caliper. That's another thought too.
 

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
4,060
Reaction score
5,896
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
If plugs aren't available, they can be made out of a brake line fitting. Use a torch and fill the hole with brass.

Or use these to pinch off hoses.

If I don't have plugs I just pound an old line shut with a hammer,fold it on itself pound it shut again. I'm too lazy to break out the oxyacetelyne.
 

Attachments

  • Screenshot_2024-02-18_192059.jpg
    Screenshot_2024-02-18_192059.jpg
    58.1 KB · Views: 20

Radiohead

Reducing tire diameter 1000 revolutions at once
Joined
Mar 17, 2022
Posts
1,815
Reaction score
5,915
Location
Low Earth Orbit
First Name
Eric
Truck Year
MCMLXXIX
Truck Model
C-20 Silverado Camper Special
Engine Size
454 crazy cubes, or 7.4 luscious litres
When you replaced the master cylinder, wasn't there plastic plugs in the ports? Those will work for testing purposes.
 

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
4,060
Reaction score
5,896
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
When you replaced the master cylinder, wasn't there plastic plugs in the ports? Those will work for testing purposes.
Have you ever tried this? I always assumed there was no way those plastic threads would hold that kind of pressure,but never tried.
 

hoagster

Full Access Member
Joined
May 16, 2017
Posts
1,303
Reaction score
3,258
Location
California
First Name
Mike
Truck Year
1973, 2000
Truck Model
K10, 1500
Engine Size
5.3l
15 psi, or 1 bar will let you know you have a leak somewhere!
 

Radiohead

Reducing tire diameter 1000 revolutions at once
Joined
Mar 17, 2022
Posts
1,815
Reaction score
5,915
Location
Low Earth Orbit
First Name
Eric
Truck Year
MCMLXXIX
Truck Model
C-20 Silverado Camper Special
Engine Size
454 crazy cubes, or 7.4 luscious litres
Have you ever tried this? I always assumed there was no way those plastic threads would hold that kind of pressure,but never tried.
I have metal fittings plugged for when I am doing this operation, but light pressure is all you need to do a quick test, so the plastic plugs should be sufficient.

One thing I've not seen discussed yet is whether or not the fluid level in the master stays the same after several dozen pumps with the engine running. My thought process wonders if the master is leaking out the piston seals and into the booster. Tbe odds are really slim, but it is something to consider if the pressure check comes up empty.
 

Forum statistics

Threads
42,087
Posts
908,642
Members
33,560
Latest member
bgeise
Top