brakes gone then back then gone
 

drove out this morning everything normal and good.
started back tonight and brake pedal dropped to floor...
First 80% of travel nothing happens, then sudden engagement of what feels like rear brakes only as pedal gets close to floor.
Then next second full normal brakes again. abs and brake lights coming on and off but not necessarily in conjunction with full or partial braking power.
drove for about 40 min... sometimes had full brakes sometimes what must be rear only.

is this some sort of valve malfunction? can anyone point me in a direction for diagnostics?

recent changes: installed new block heater & added front diff fluid, but can't see how these would matter unless maybe some kind of voltage leak from new heater fried something?

thanks

'97 5-sp manual 4x4

brakes gone then back then gone
 

drove out this morning everything normal and good.
started back tonight and brake pedal dropped to floor...
First 80% of travel nothing happens, then sudden engagement of what feels like rear brakes only as pedal gets close to floor.
Then next second full normal brakes again. abs and brake lights coming on and off but not necessarily in conjunction with full or partial braking power.
drove for about 40 min... sometimes had full brakes sometimes what must be rear only.

is this some sort of valve malfunction? can anyone point me in a direction for diagnostics?

recent changes: installed new block heater & added front diff fluid, but can't see how these would matter unless maybe some kind of voltage leak from new heater fried something?

thanks

'97 5-sp manual 4x4
__________________

Brake fluid check out?

Master cyl? try pinching off all hoses at the wheels, check pedal. release one at a time and check pedal to isolate the problem. I also had a similar situation on a customers chev 3/4 ton a while back. Pulled all 4 wheels as i thought there was a leak. Turns out the lining on the aft brake shoe became separated from the metal and was slipping around causung the pedal to go to the floor sometimes and others to be rock hard. Good Luck.

fluid is good.
what a weird condition with the separated shoe. i would suspect unusual noises in a case like that? my rear brake shoes and drums are pretty new, but i'll take a look. also, if my fronts were working normally i don't think there'd be such a noticeable difference. i have that thing with the rear proportioning valve not working right. i burn through front pads but my drum shoes last forever. thought that might be related but not sure.


is there an electronic piece controlling abs that could be malfunctioning?

Checked PS fluid or brake fluid lately?

Looked at backside of tires for fluid leakage from calipers or brake lines?

One or both of the wheel speed sensors are out.

Well, if you have checked your brake fluid levels ( and I am sure you have ), you next best bet is the ABS valve. Look under your hood and see if you have 2WABS or 4WABS. If you have 2WABS, there is a reservoir with solenoid valve that allows the computer to bleed excess brake pressure into the accumulator, when the condition is rectified, it allows the fluid to go back.

Both my 97's have been 2WABS and I have had issues with both. I seem to remember they were the same from at least 94-97 ( perhaps 99 ) and gasser and 1500 used the same valve, super easy to find in the junkyards.

If you have 4WABS I can't help you other than to say it is probably the problem.

Never pinch off brake hoses for testing unless you want to replace them.

When the brakes are working fine and applied does the pedal creep down slowly?
If so the problem is an internal master cylinder leak.

yes pedal feels weird in both directions.
at first, it sinks almost to floor and gets stiff as rear brakes only come on.
fairly slow for full return of pedal to top
then usually after pumping pedal 3-4 times, and esp if i pull the pedal back up with my foot, fronts come back on and pedal/breaking feel normal

I'm having this exact same issue. Best of luck. Ill be following to see what you find.

ok, based on not knowing how to diagnose more specifically, i've gone ahead and pulled the master cylinder out and intend to replace with a new one.

1st question: when i depress the plunger by hand, fluid shoots out both brake line holes. this is not a definitive sign that the master is working correctly, right?

2nd: napa parts seem to often be cheap chi-won-ese junk. am i better off going to the dealer for this... considering functional brakes could be a life/death kind of part?


thanks

Dealer probably wont have it in stock and will be at least quadruple the cost. If you can live with that it will probably be a better part. I've been having good luck with autozone parts honestly, used to be Napa. Last thing I got from Napa was a reman steering box....paint was still tacky and steering had a tight spot every turn.

On another note, my dad's old S10 blazer was having some wacky brakes, ended up being the ABS module. We used some unions/adapters/etc to jump out the abs to find that out.

Good luck with the master cylinder, I tried 2 Raybestos new units because I just knew it was the MC.

It wasn't. I order mine from Rock Auto and stick with Raybestos.

thanks patdaly. if i'm looking at the right thing, that abs valve is $1044 at rockauto? did you have to replace yours or can it be repaired? that's not a cheap part!