97 Coupe A4, Brakes are spongy and if you hold firm pressure on pedal with engine running pedal will sink almost to the floor. With engine off, pedal is semi-hard and with firm pressure you can feel sink. Both running and not, you can feel pedal sink.
Pedal sinking is the most pronounced with engine running. And during firm braking, pedal is literally on the floor.
It seems that the rear brakes are not fully engaging, by looking visually on new rotors for wear and low heat.


Parts replaced in sequence.
Rotors and pads
Front Calipers
Braided lines
Master Cylinder
Booster Check Valve
Bleed brakes with pressure method, manual method. (no change)
Bleed brakes with Tech 2 Auto Bleed procedure (all valves and pumps and each wheel bleed correctly. Road test. (No change)
Replace rear calipers and pads
Bleed brakes using manual and pressure method (No change)

I have bleed literally gallons of brake fluid thru system and do not see any more air in system and nothing changes. ( I did not bleed with Tech 2 after rear caliper install)

Today, I tried with engine running and booster check valve removed and blocked to the engine, the brake pedal was hard and held firm.

Help, Looking for suggestions, I’ve been working on this too long.
I am thinking maybe bad master cylinder and proportioning valve, but now maybe booster?
DTC codes P1517H, B2264H, B2263H, B2265H, B2860HC

Thanks in advance

 

If there is no sign of leaking fluid -- which I don't see any mention of -- it pretty much has to be the master cylinder, where fluid can bleed past pistons without coming out. I don't know for certain how the BPMV works, and whether it can cause a similar problem. You note you changed the master -- GM OEM replacement or other? Every difficult to fix soft pedal I've ever had ended up being the master. On my pickup, I had an unending soft pedal. Fabbed a line from the front master fitting to the rear master fitting -- still spongy with no where for the fluid to go. Autozone master no better. New Toyota master, no more problem. Other than a much lighter wallet.

 

Thanks redzg

I replaced the master cylinder with a 97 specific one from Ekler's I think. I will check with GM and see if it is still available. I was told any year will work on the 97, not sure. The shotgun approach to fixing this is not working well for me.

 

Brake booster or a vacuum leaking, other rear calipers leaking fluid out the seal. Last but not least test abs pump. Installed booster valve wrong ?

 

I’ve also learned just because it’s new doesn’t mean there can’t be a problem with new products.

 

Far too true!

 

i vote brake booster

 

indeed sounds like a brake booster issue !

is It a 6 spd or auto ?

 

Auto

 

What was ur bleeding sequence ? I’ve performed a tpms sensor reprogram in similar sequence from left driver to passenger to rear passenger to driver rear . Is also the bleeding order. If u did not follow this sequence u could have trapped air.

 

Calipers upside down?

 

I will double check calipers, Bleed sequence was LR RR LF RF for the 97.

I'm at the point now to double check all the obvious, and then change the booster ,MC and then a good bleed and finish up with Tech 2 bleed. I will check with my local corvette shop to see if he wants to tackle it. Not sure if my back wants to tackle the booster.

Thanks

 

I found this from a old thread actually

1997-2000 LR/RR/RF/LF.

 

The latest, picking up car today from local ind. repair shop and also verified by local Corvette repair shop. they both say EBCM is bad. The thing is I have no abs/tcs codes or lights and tcs works and turns off and on. Also, when activated the abs works and I get abs active on steering wheel. They say the pump side is bad even though it checked out with Tech 2.

Question is, Does this sound logical? I know I will probably never find one for the 97. Suggestion?

Thanks in advance

 

Wait wait wtf is going on ?

 

Just stumbled onto this thread, hope you don't mind me stopping by.
My other car is a 2001 Audi S4, which I have some similar symptoms to you and I've had a very similar parts shotgun approach (entire brake setup, master, lines, GALLONS of fluid). In case you care to read my on-going saga: https://www.audizine.com/forum/showt...dead-zone-quot

On my car as potentially with you & your car, the next move is to replace the ABS module/pump (I think it's comparable to the EBCM?). My current theory is there's a clog in one of the solenoids or something and air is trapped that can't leave when pressure bleeding or cycling the pump. A little light reading on that: https://www.jaguarforums.com/forum/x...-valve-188478/
BUT that theory is based on the fact that when I cycle the ABS pump, fluid only briefly exits the FR & RL while fluid flows freely from FL & RR. During other bleeding fluid flows just fine.

 

Since your at why not replace the rear calipers lines re bleed and see where that gets ya. Inspect the brake booster line from the intake manifold tighten if need to be .

 

Good read MetalMan2, the S4 sounds just like my C5. I have a little better pedal when it came out the shop today. Shop says ABS pump (ECBM) is bad, wish I had a tech2 to run all checks on ecbm. I've been messing with this for about a year now. Thanks for the info.

 

Speedy007, I did replace rear calipers and was bleed with Tech2 at shop. I will double check booster line to intake manifold and see,
Thanks for the help.