brake issues

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.

RLC2020

Junior Member
Joined
Oct 13, 2020
Posts
16
Reaction score
18
Location
Malabar FL
First Name
Ross
Truck Year
1983
Truck Model
Suburban C20
Engine Size
6.2L Diesel
Hi all,

Issue is when at a stoplight for an extended period of time, the pedal will slowly inch down while holding it and the brake light will come on if stopped long enough. Once I let off the brakes they are fine again. The brakes all seem to work great and the truck stops just fine even towing my 5,000lb boat. I had to do a panic stop the other day and I was actually impressed with how quick it stopped. Any suggestions? This is my daily so I don't want to tear it all apart without having replacement parts ahead of time if possible.

Brake fluid level in the master cylinder is full, clean and has not moved. I don't see any wet spots on the back of the drums either so I don't think I have a leak anywhere.
 

Curt

Full Access Member
Joined
Sep 11, 2017
Posts
676
Reaction score
1,140
Location
Loco Hills
First Name
Curt
Truck Year
1984
Truck Model
K-30
Engine Size
383
Stock brakes?If so,most likely your combination valve on the back of the forward most cross member.

So people have said you can slam on the brakes and reset it.Can’t vouch for that.

I would look for leaks,check master cylinder level,then bleed brakes.If it persists,change the combo valve.If that doesn’t work,probably start getting into the brakes themselves.

If it is a stock setup,if you go after a combo valve,make sure it’s disk/drum and not disk/disk
 

Craig 85

Full Access Member
Joined
Jan 25, 2017
Posts
3,889
Reaction score
4,024
Location
Nashville, TN
First Name
Craig
Truck Year
1985
Truck Model
K30 SRW
Engine Size
454/TH-400/NP205
I would say master cylinder. I had this happen to me 40 years ago right after I started driving. I was sitting at a red light and the brake pedal started inching towards the floor. I lost enough pressure that the car started rolling into the intersection. I was able to pump the pedal, but lost pressure again and had to apply the parking brake to keep it stopped till the light changed.
 

fast68chevy

I am the original fast68chevy
Joined
Oct 17, 2020
Posts
172
Reaction score
88
Location
LINCOLN IL
First Name
RANDY
Truck Year
1975
Truck Model
C20
Engine Size
350
def something going on with the M/C if fluid is no disappearing. pistons seals in the M/C are toast. is the booster end of the M/C wet with fluid?
 

GXPWeasel

Full Access Member
Joined
Nov 5, 2019
Posts
573
Reaction score
1,033
Location
Kansas
First Name
Greg
Truck Year
1982 | 2015
Truck Model
Sierra C15 | Silverado Z71
Engine Size
5.0 | 5.3
Yep, master cylinder. My other guess would have been leaking into the booster, if you didn't see any leaks on the ground, but fluid level was low. This has happened to me before as well.
 

idahovette

Full Access Member
Joined
Aug 7, 2016
Posts
6,448
Reaction score
13,779
Location
Weiser Idaho
First Name
Perry
Truck Year
1975-1979
Truck Model
K20-K10
Engine Size
350
AND DON"T get a reman or rebuilt, just buy a NEW one!!!!
 

highdesertrange

Full Access Member
Joined
Dec 19, 2013
Posts
1,066
Reaction score
211
Location
kalifornia
First Name
mike
Truck Year
1978
Truck Model
k30
Engine Size
454
yep master cylinder is bypassing internally. highdesertranger
 

RLC2020

Junior Member
Joined
Oct 13, 2020
Posts
16
Reaction score
18
Location
Malabar FL
First Name
Ross
Truck Year
1983
Truck Model
Suburban C20
Engine Size
6.2L Diesel
So it seems that Master Cylinder is the place to start. I'll order one of those. Where are you all getting your parts? I'm sure there is a preferred vendor most of you use. I want to make sure I get OEM for something like this for sure.

I'm going to refresh everything in there while I'm at it. New pads/shoes, calipers, wheel cylinders, new fluid, etc. Any other suggestions? I haven't done drums in many many years. Can I reuse the springs and hardware, or should I order a new kit for that as well? Thank you for all the responses.
 
Last edited:

Goldie Driver

Full Access Member
Joined
Mar 19, 2018
Posts
3,924
Reaction score
6,375
Location
Houston, Texas
First Name
Britt
Truck Year
1980
Truck Model
GMC K1500 Suburban
Engine Size
350
So it seems that Master Cylinder is the place to start. I'll order one of those. Where are you all getting your parts? I'm sure there is a preferred vendor most of you use. I want to make sure I get OEM for something like this for sure.

I'm going to refresh everything in there while I'm at it. New pads/shoes, calipers, wheel cylinders, new fluid, etc. Any other suggestions? I haven't done drums in many many years. Can I reuse the springs and hardware, or should I order a new kit for that as well? Thank you for all the responses.

I agree with new on the brake stuff all the way.

I just went with the Advance Auto stuff, though - if there is an Xcel auto parts around you might find AC Delco, but my 35 dollar ish one works fine.

The drum hardware kits are pretty cheap, and the springs do lose tension with heat and time, so those , and wheel cylinders, would be a good call.

You can change wheel cylinders with the removal of 1 brake spring and the shoes still on, BTW.

At least on my half ton.
 

RLC2020

Junior Member
Joined
Oct 13, 2020
Posts
16
Reaction score
18
Location
Malabar FL
First Name
Ross
Truck Year
1983
Truck Model
Suburban C20
Engine Size
6.2L Diesel
^ I'm going to do O'reily since they can get the parts next day. I was on a few websites but they were all 1 week plus and I need to tow this weekend with it. Any idea on bore size for the master cylinder? Looks like there are 2 that say they will fit my truck. 1.19" and 1.25"

my vin#1G8GC26J4DF155651 if that helps.

I couldn't find the brake code on the tag under the hood.

Since I'm diesel I guess I have the hydroboost set up. Anything I need to know with that? I've never replaced a master cylinder, but I'm going to learn as I go.
 
Last edited:

Blue Ox

Turning Diesel Fuel Into Fun
Joined
Mar 27, 2018
Posts
4,831
Reaction score
10,631
Location
LI-NY
First Name
Derek
Truck Year
MCMLXXXV
Truck Model
K20HD
Engine Size
6.2L
Any idea on bore size for the master cylinder? Looks like there are 2 that say they will fit my truck. 1.19" and 1.25"

my vin#1G8GC26J4DF155651 if that helps.

You can probably sort that out with drum size. C6P heavy duty would be 13" drums and the larger master. Standard would be 11" drums and a 30mm master.
 

Goldie Driver

Full Access Member
Joined
Mar 19, 2018
Posts
3,924
Reaction score
6,375
Location
Houston, Texas
First Name
Britt
Truck Year
1980
Truck Model
GMC K1500 Suburban
Engine Size
350
^ I'm going to do O'reily since they can get the parts next day. I was on a few websites but they were all 1 week plus and I need to tow this weekend with it. Any idea on bore size for the master cylinder? Looks like there are 2 that say they will fit my truck. 1.19" and 1.25"

my vin#1G8GC26J4DF155651 if that helps.

I couldn't find the brake code on the tag under the hood.

Since I'm diesel I guess I have the hydroboost set up. Anything I need to know with that? I've never replaced a master cylinder, but I'm going to learn as I go.


Pretty straightforward - you will need flare wrenches for the lines to the MC , and to bench bleed it before installation. I *think* the new MC's come with instructions on how to do it, and eons ago they came with plastic thread in fittings and rubber hoses to flop back into the MC.

I still have my old ones as my new ones did not come with - cheap ass Chinese bastages ...

And, yes, good luck finding US made parts. I was shocked that my BW ignition module was made in the US, but no telling how long it sat on the local Advance Auto's shelf. :Big Laugh:

Don't know where it was made, but they stocked the floor switch for the dimmer, too ,which was good since my OE died - at night. :crazy:
 

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
2,017
Reaction score
1,808
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
Since it’s a C20, do you have a 14 bolt FF rear axle?

If so, the rear drums are a much bigger job. With a FF axle you have to remove the axles before you remove the drums. Not really that hard, but a lot more involved than just slipping the rear drum off like you do on a light duty axle.

Bruce
 

Latest posts

Forum statistics

Threads
41,848
Posts
903,425
Members
33,362
Latest member
Dhatch84
Top