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Brake Pedal to the floor after new calipers, bleed, flush and auto shop bleed

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BigDaddyBruin

Full Member
Joined
Nov 29, 2007
Messages
21
My brake pedal was working fine 2 days ago. I FINALLY got around to executing my front brake upgrade to the GMT900. I put in new Centric calipers, bled at all four wheels - making sure I never let the brake fluid reservoir get below the MIN line. The brake pedal went goes to the floor. Feels like braking with a truck from the 1970's.

Took the car to the local Big Brand Tire & Service shop to flush the brake lines/fluid with the pressure flush, and replaced with all new DOT-4 fluid. After all new fluid, and bleeding at all wheels, the problem is still there - pedal is STILL down to the floor to get the car to stop.

The mechanic suggests replacing the Brake Master Cylinder. How can I really tell if the Master Cylinder is dead because the fact it goes out at the same time I put on new front calipers feels like too much for a coincidence. Anything else I should consider? Anything else I should try?
 
Check the fluid level.
 
krazycarguy said:
I really hope you didnt use dextron fluid in your braking system....
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Thanks for the catch... I corrected to DOT-4, not Dextron  :laugh:

MS03 2500 said:
Check the fluid level.
Click to expand...
Reservoir shows Brake Fluid level at MAX line.
 
A bad master cylinder usually gives a pedal that will gradually drop to the floor after braking, i.e. stopped at a light, the vehicle will start to crawl and you'll have to increase pressure or pump the brake. Sounds like the problem is not being caused by the master cylinder but by the calipers. The caliper pistons and master cylinder piston MUST operate as a matched set for volume of fluid displacement.

Are you certain the calipers are an exact match for the vehicle, i.e. the master cylinder used in your Avy???  Do a couple of quick pumps bring up the pedal and it remains firm under pressure?

If the volume of brake fluid displacement for the new caliper pistons to expand exceeds that of the old calipers, the pedal will go lower because you only get one stroke of a given volume displacement from the master cylinder's in line pistons no matter how much fluid is in the reservoir...a larger volume of the caliper pistons will lower the pedal, too much and the pedal will hit the floor....if so, either go back to calipers with piston displacement similar to the old volume or get a master cylinder with a larger bore piston so the displacement matches that of the calipers pistons.......

Replaced and added to my original  post:

If your brake specialist agrees that the above is causing the problem, and you decide to replace the master cylinder rather than going back to the old calipers, you will also have to make certain that the proportioning valve assembly is compatible with the new arrangement.
 
Thomcat - you are 100% correct in your diagnosis! It is not the Master Cylinder. Painfully, I took the Avalanche to the Chevy dealer. After $110, they identified the problem... washers! Yet those bastards wanted to charge me $385 to replace washers and re-bleed the brakes. But that is a story for another time...

Long story short - I left the washers from the OLD calipers on the brake fluid line where it attaches to the calipers, and they are NOT a match to these new calipers (rounded triangular vs. these NEW calipers just want ROUND washers). Even worse, my buddy who did the other side ADDED the new washer on top of the old. So there is not a good seal where the brake lines meet the calipers. So replaced the washers, used my new MityVac to solo bleed the brakes, and they are now better than EVER!

Regarding whether the "Master Cylinder might need to be upgraded" also... that is a very good thought but even though the calipers are an upgrade ('02 Avalanche GMT800 upgraded to GMT900 brakes), the new calipers are not so large (from a fluid capacity perspective) that the current Master Cylinder can't handle it. But again, smart suggestion.
 
:help: :help: :help: I had hoped just replacing the OLD washers from my GMT800 calipers with new washers AND brake line screws that came with my new Centric GMT900 calipers would do the trick... and I was WRONG - AND SO WAS THE CHEVY DEALER!

Once I again bled the brakes (with the engine off), the brake pedal is nice and firm. Then, when I turned the car on and depress the brake pedal, it goes totally to the floor once again providing MINIMAL braking power. There are no leaks of brake fluid anywhere.

It seems it has to be that the Vacuum Booster on the Master Cylinder somehow went bad when I bled the brakes originally, and I am planning to have it replaced to the painful cost of $385. But I am thinking perhaps before I do that, I should put the OLD calipers and rotors back on and see if the brake pedal responds properly. If so, then I know that I have gotten some bad new calipers.

Any other suggestions?
 
BigDaddyBruin said:
:help: :help: :help: I had hoped just replacing the OLD washers from my GMT800 calipers with new washers AND brake line screws that came with my new Centric GMT900 calipers would do the trick... and I was WRONG - AND SO WAS THE CHEVY DEALER!

Once I again bled the brakes (with the engine off), the brake pedal is nice and firm. Then, when I turned the car on and depress the brake pedal, it goes totally to the floor once again providing MINIMAL braking power. There are no leaks of brake fluid anywhere.

It seems it has to be that the Vacuum Booster on the Master Cylinder somehow went bad when I bled the brakes originally, and I am planning to have it replaced to the painful cost of $385. But I am thinking perhaps before I do that, I should put the OLD calipers and rotors back on and see if the brake pedal responds properly. If so, then I know that I have gotten some bad new calipers.

Any other suggestions?
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Assuming the brakes were bled properly and there are no leaks there appears to be a mismatch in volume (incidentally the same problem can be seen sometimes when replacing a power booster if the push rod in the new booster is a mismatch, too short). The fluid capacity in the master cylinder reservoir has NOTHING to do with the volume ejected on a single stroke of the piston which is not a lot considering that there are two separate in-line pistons within the master cylinder and only one pumps fluid into the four pistons on the two stock GM calipers with a single stroke........a difference in volume of a single ml. in each piston can make a big difference.

Doesn't sound like a bad brake booster......hard to see how you could damage it bleeding the brakes?????  Usually the test to see if the vacuum booster works (assuming you have a stock vacuum booster and not a hydraulic booster running off the power steering pump) is exactly that.....pump the pedal a few times with the engine off and the pedal rises and is firm to show there are no leaks or air.....turn on the engine to generate vacuum and if the booster is GOOD the pedal will drop and give an assist.......a bad booster (or check valve within the booster at the end of the vacuum hose) will usually hiss, cause a rough idle or give no boost. Another thing to check is the eyelet bushing where the brake pedal attaches to the brake rod......although unlikely, if cracked, missing or compressed it would cause the pedal to drop.

You are correct in your assumption to replace the original calipers as a diagnostic. Unfortunately ...that is the ONLY way you can be sure the problem is not being caused by a mismatch of the calipers to the rest of the braking system. If the pedal does raise.....keeping the old calipers may be the best solution. Being that brakes determine how long you and others may remain on this Earth I suggest that if the stock calipers solve the problem....leave them on there.......you could get away with putting larger brakes on the front in pre '65 days where a single piston master cylinder directly powers all wheel pistons........but now we are talking dual piston master cylinder, dual diagonal braking, proportioning valves, antilock high pressure pumps, complex hydraulics, etc....changing just one portion of a complex systems could cause problems.

If it is a volume problem it MAY be cured with a master cylinder of larger volume on the stroke.....but consider you may also have to change the booster or at least the push rod within the booster if the new cylinder has different internal dimensions, shorter of longer depth hole within the cylinder......also if you upgrade the master cylinder you may then have problems with the rear calipers unless you upgrade them also.
 
What year AV do you  have.  ???
 
Thomcat - Great suggestions which I am still going to consider getting doing a larger bore MC with Hydroboost mod later, but I think discovered something last night that no one (including the brake service shop and the local Chevy "stealership") noticed or guessed... :laugh:

While I am more than a little embarrassed to admit this, my experience might help someone else... I think I inadvertently switched the right and left calipers and installed each on the wrong side. Why do I think this... because the bleeder screws are at the BOTTOM of the calipers - not the top where they are supposed to be.  So no wonder I am getting no air bubbles on my bleed... because the air is "trapped" at the top of the caliper and the fluid is denser so it exits the caliper from the bottom not allowing the trapped air to escape.

MS03_2500... I have a 2002 Avy 1500 Z71 with newly (albeit "wrongly") installed GMT900 Mod using Centric calipers & Eline Drilled Rotors and Pads for a 2007 Tahoe.
 
If you mix the DOT 4 with the DOT 3. Or how well did you flush it.
 
BigDaddyBruin said:
I think I inadvertently switched the right and left calipers and installed each on the wrong side. Why do I think this... because the bleeder screws are at the BOTTOM of the calipers - not the top where they are supposed to be.  So no wonder I am getting no air bubbles on my bleed... because the air is "trapped" at the top of the caliper and the fluid is denser so it exits the caliper from the bottom not allowing the trapped air to escape.
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That would do it....I think you solved your problem
 
MS03 2500 said:
If you mix the DOT 4 with the DOT 3. Or how well did you flush it.
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The brake shop I took it to did a "power flush". They hooked up a machine with a "cap" to the Brake Master Cylinder Reservoir, put tubes on all 4 bleeder screws, opened them, and ran new DOT4 fluid until all 4 screws let out nothing but clear brake fluid. It was actually pretty neat for me to see.
 
How did they miss the calipers were on the wrong side.
 
BigDaddyBruin said:
The brake shop I took it to did a "power flush". They hooked up a machine with a "cap" to the Brake Master Cylinder Reservoir, put tubes on all 4 bleeder screws, opened them, and ran new DOT4 fluid until all 4 screws let out nothing but clear brake fluid. It was actually pretty neat for me to see.
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Your thoughts were correct...the bleeder valve must be at the highest point of the piston bore within caliper...under conditions with the bleeders on the bottom all the air would not be removed from the system.......this should have been picked up by the person doing the bleeding.
 
Interesting stuff, I might add always check the washer seat on rebuilt calipers. I just replaced both front calipers at 218000 miles and one of the replacements had a dimple in the washer seat area and would not bleed. Could've been a real bear for someone not used to working on cars to find.
 
I still can't figure out how the mechanic missed it while bleeding them, I would find a new one.
 
As I said in a previous post "Assuming the brakes were bled properly and there are no leaks..." and we know what happens when we ass u me.


The first and mandatory step after bleeding is to check for a firm pedal. There is no way that the pedal was firm with the engine off after bleeding.....find a new mechanic.
 
So both a brake shop and dealer did not catch this? And the dealer told you the washers were wrong.. So they obviously looked at them closely..

Don't know that I would trust either that dealer or the brake shop...

Rodney
 
I'm having the some problem. Replaced the front driver side caliper on my 02 av. no peddle at all. I know it's the correct caliper.arrrrg
 
Let's hope you did not put the caliper in upside down.  LOL.

Did you bleed the system

 
pittfan said:
I'm having the some problem. Replaced the front driver side caliper on my 02 av. no peddle at all. I know it's the correct caliper.arrrrg
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Suggested Checklist:
1.  Bleeder screw located at the highest point of the caliper  :E:
2.  You replaced both washers on the brake line screw with new washers that came with the caliper
3. When bleeding, you removed the cap on the reservoir to enable the flow of fluid
4. You bled at least BOTH front calipers, but preferably all four
5. The brake line screw is tight

Let us know how you did.

 
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