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RICK, ASE Certified Technician
Category: Chevy
Satisfied Customers: 426
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1991 Chevy: push the pedal..master cylinder

Customer Question

We are looking at buying a used 1991 Chevy c1500 Silverado. This truck basically has NO brakes when you push the pedal with the truck started, off it feels like there might be a bit more but not much. We changed out the master cylinder and the proportion equalizer today and no improvement at all. Please could you tell us what we might be over looking as we do not know where to look next? Any help would be appreciated.
Submitted: 8 years ago.
Category: Chevy
Expert:  RICK replied 8 years ago.

hi Customer,

any fluid leaks? known for brake lines rotted and leaking, someone might have pinched off lines to wheels, usually rear lines, can you get wheels off ground and apply brake pedal and make sure all wheels stop. also hoses collapse and no fluid gets to wheels. When you chaged parts today, did you have any problems bleeding air from all 4 wheels?

Customer: replied 8 years ago.

As I stated in the origial "already tried" portion of my post:


Yes, we checked all lines, no leaks. No we did not have problems bleading any of the areas, all 4 wheels. The fluid coming through when all air was removed was new fluid, so it is definitely getting through the lines. We did not see any difference in the fluid levels when pumping or pushing on brakes after all bleeding was done either, so no leaks in any of the lines and nothing has been pinched off? Are all 1991 c1500 models ABS brake systems? We did not see any evidence of this truck having ABS on the dash control panel and no ABS emblems on the dash or outside of the truck anywhere?

Expert:  RICK replied 8 years ago.

Hi Customer,

if you pump pedal , with engine off does pedal get harder, all 1991 c1500 did not have antilock brakes , if you give me vin number can look at options to see if built with antilock, with the power assist on this vehicle , booster will let you push pedal all the way to floor, if you drive vehicle, how far down does pedal go on normal stop, how about rear brakes are they out of adjustment, here is diagnosis chart.....




A brake pedal that sinks all the way to the floor is the result of the master-cylinder not developing any hydraulic pressure. This can be caused by a severe fluid leak which has emptied the master-cylinder, or by complete failure of the seals inside the master-cylinder.
A pedal that slowly sinks to the floor is the result of hydraulic pressure leaking off. This may be from an internal leak in the master-cylinder or an external brake fluid leak.

NOTE: Rear brakes which are extremely out of adjustment in conjunction with air in the hydraulic lines can also produce this symptom. Always verify that the shoes/linings are properly adjusted and the brakes have been bled prior to replacing a master-cylinder.


  • Verify that the fluid reservoir is full.
  • Verify that there are no signs of leakage at the area where the master-cylinder bolts onto the brake booster. If any leakage is indicated, the master-cylinder should be replaced.

NOTE: Minor leakage will often be indicated by "bubbled up" paint on the brake booster just below the master-cylinder. Brake fluid is very corrosive to painted surfaces.

  • Inspect the rubber gasket on the inside of the master-cylinder lid. If the gasket is swollen or bloated, the brake fluid is contaminated with oil.
  • Siphon or scoop a small amount of brake fluid out of the master-cylinder and place into a styrofoam cup filled 2/3 of the way with water. Brake fluid will mix with the water while oil contaminants will float on the surface and dissolve the styrofoam.

NOTE: If any oil contamination is present ALL brake components containing rubber seals will need to be replaced or overhauled. This includes the master-cylinder, both front calipers, both rear wheel cylinders, and all flexible brake hoses.

  • Trace all brake lines from the master-cylinder to the calipers and wheel cylinders. Replace any lines that shows evidence of leakage.
  • Remove all wheels and inspect the calipers and wheel cylinders for brake fluid leakage. Replace any component that shows evidence of leakage.
  • If no signs of external leakage can be detected, bleed the brakes and verify the shoes/linings and emergency brake are properly adjusted.
  • If the pedal still sinks to the floor, replace or rebuild the master-cylinder (Non-Antilock Brake Systems only, For ABS systems see Antilock Brake Systems/Testing and Inspection).
Customer: replied 8 years ago.

When driving the truck, the brakes go very close to the floor with a very spongy like feel. No quick stopping that is for sure, very unsafe to drive as it basically has No braking power.


When looking at the master cylinder, the isolation dump valve below is long with the control module on the right side of the dump valve. Looking in the Haynes Repair Manual covering this year model vehicle this gives me the indication that this very well could have RWAL. There is also a brake light that does not seem to have anything to do with the parking brake that comes on when the truck is running and while driving (what little we drove it). This light is the telling us there is a problem in the RWAL system, is that correct? According to what I have read in the Haynes Repair Manual and the diagnostic chart you just sent it sounds like that is where the problem is, the RWAL system. So if I am understanding everything this is a repair best made by a professional, as without the "special tools" this can not be fixed even by a seasoned DIY mechanic.? I say seasoned as we have worked on numerous cars and trucks 1991 and older without the ABS systems without any problems, including changing out master cylinders and proportion valves, pads and all. Can you tell me if the ABS systems is really that much more difficult to work on? Or do you think this is more of a hydralic problem where we should be looking more at the adjustment of the rear brakes? Is there a diagnostic chart for ABS systems and testing them?

Expert:  RICK replied 8 years ago.

hi Customer,

system not that difficult, heres how to read codes, would check rear brake adjustment, get back to me on codes will send you charts..graphic



Diagnostic Trouble Codes (DTC) 's can be displayed by jumping terminals "A" and "H" of the Data Link Connector and observing the flashing of the "BRAKE" warning light. This should only be done when the brake lamp is "ON".
The terminals must be jumped for about 20 seconds before the code will begin to flash. Count the number of short flashes starting from the long flash. Include the long flash as a count. Sometimes the first count sequence will be short. However, subsequent flashes will be accurate. If there is more than one failure, only the first recognized code will be retained and flashed.


  • DTC's 6,9, and 10 will only turn the "BRAKE" lamp on while the fault exists. These are soft codes faults and can only be read with the Tech 1 scan tool.
  • Do not jump terminals "A" to "H" if the "BRAKE" lamp is not on. Jumping the terminals with the "BRAKE" lamp off will set a false DTC 9. This will turn the "BRAKE" lamp on and disable the antilock system.


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Customer: replied 8 years ago.
Thank you very much. I do appreciate any charts or information you could send me. We are trying to decide if we are going to buy this truck for my 16 yr. old son. Right now the brakes are not safe for anyone to drive it home, so unless we can spend a lot less time than we did today (5 1/2 hrs.) chasing parts and actually working on it we may or may not buy this truck. This is why I have not sent a vin number as I do not have it yet, so unable to verify for sure if it has RWAL. However the picture I am currently looking at in the Hanes manual is exactly what the RWAL system components looked like on the truck. So if I can get the codes so when we drive back out "to the sticks" where this truck is tomorrow and check the codes we know what to do to fix it, that would be greatly appreciated. Thank you again for all your help and information, it is greatly appeciated. I'll let you know what worked.
Expert:  RICK replied 8 years ago.
your welcome, pleaure to try and help... rick