Kyle Ray

so, i've searched the archives a bit for some of the symptoms that i'm having and found a couple that kinda fit, but i have a specific question that i didn't find. here's the back story.

braking has been fine but every once in a while (especially if i had been driving it for an extended period of time) i would push the brake pedal and it would go to the floor. if i pumped it rapidly it would build pressure and work-again-off-again until i got to my destination. the next time i drove it no problems right away and if it was a short trip, no issues at all. i knew that i was going to changing my pads soon, so when i did that i bled the brakes with a power bleeder.

i'm thinking my bleeder is not functioning properly because i can see air bubbles entering the system where the hose that goes to the reservoir cap, is clamped to the external port at the shoulder of the tank. I don't notice this until i'm nearly done and when i do finish the brake pedal is spongy. i did NOT bleed the MC. i figured i have air in the line somewhere and decide to do the bleed with the 2 man technique.

the following week we do the 2 man bleed and after reading some old posts we start at the front and finish in the back because of the double piston setup of the cylinder. what we find is that when we pump up the brakes and then open the bleed valves on the front calipers, nothing comes out, just a drop and there is no additional pedal movement. when we do the rears, pump up the pedal, open valves; fluid shoots out and the pedal drops another inch.

i know that the lines are clear because if we have a front valve open and pump the brakes the fluid shoots out as well.

Does this sound like a bad MC? i'm wondering if the front piston seal in the cylinder is failing and the fluid is pushing out behind it instead of pushing the piston forward. but i also wonder if the rear piston in the MC would be actuated (which it is, as evidenced by the fluid and pedal movement when opening the rear valves) if that front MC piston wasn't sealing. so, i guess i'm not totally convinced. we also checked the booster and it seems dry.

thoughts?

Mrmerlin

If you have an original master cylinder replace it . with a new ATE part its about 265.00
Also get a Liter of ATE brake fluid, about 18.00
and some Dow Corning 111 . about 16.00
Get a set of stainless steel flex lines and swap them in.
NOTE if on removal of the old MC you find fluid leaking into the booster it would be a good idea to swap in a booster , about 286.00
NOTE I know all of this stuff sounds expensive but once your brakes are working you will quickly forget how much it cost to have a good brake pedal.

use the DC111 on the rubber tank grommets where they go into the MC feed bores,
coat any bare metal with it so water wont corrode it.
This will prolong the life of your new master cylinder and prevent the grommet bores from corroding and thus letting water into the MC.

When you have the new MC installed post up and i will add in a bleeding procedure.

The Forgotten On

^^ I agree with Stan's post. If bleeding the brakes didn't fix the problem then you need to replace the master.

They corrode when not flushed every few years and can get damaged when you pump the brakes over a unused part of the bore filled with corrosion. Destroying the seal in the process.

This is a safety matter so always make sure the system is 100% and replace all parts that have or are currently failing.

Kyle Ray

Thanks Stan and Blake,

That was my initial thought and it is the original master cylinder (as far as i can tell) so i will follow your advice and replace the bits.

would you recommend removing the wheel well liner for this job?

Mrmerlin

The Forgotten On

It's not needed. Nothing you need to access is hidden behind them.

OTR18WHEELER

Manual or auto?, an easy fix would be leaking grommets at the base of the reservoir, if a manual, the slave may be bad.
best to find the source of fluid leakage, and go from there.

Kyle Ray

its an auto... and that's the thing I don't see any real visible signs of a leak; at the reservoir, at the cylinder, at any of the lines or around the booster grommet. i'm hoping its the MC piston seals.

no matter if I have the car off or running, the brakes are firm (not hard) but it sinks to the floor. If running, and the pedal down, the pedal doesn't drop any more when I turn the key off. When I push the pedal I can hear a 'f'sshh' noise, not sure if this is a leak at the booster or just a normal sound. we checked the vacuum line and the connection at the hard Y-connector was poor where it attached to that 3 legged rubber splitter. There was not much suction. The suction was stronger at the connection right next to the booster. My plan was to get a new Y-connector and a new 3 legged splitter, to improve the connection points. Maybe even a new check valve for between the splitter and the booster.

How much of a pain is it to replace the booster? should I focus on the vacuum system as well, could that be a culprit?

Kyle Ray

btw, its an 86.5

upstate bob

Master cylinder fo sho. The first time the pedal flops to the floor tells you. You can change it without any cutting but it is a challenge. I recommend doing it on a lift,
not crawling around beneath. Didn't cost me much.

Kyle Ray

humpty bumpty for Mrmerlin...

Mrmerlin

get your new check valves for the booster and the blue check valve.
for the MC you can bleed it in the car ,
make sure the O ring for the MC is good and the DC111 is used on it .
Verify the booster is not filled with brake fluid if it is replace it .
You can check this with a strip of blue shop towel,
cut it then put the towel into the booster if it comes out wet the booster will have damage from brake fluid,

some might say to take it out and clean it,
but my time worth more than fussing like this, if i am taking out a booster you can bet a new one is going in.
Put in fresh parts so you dont have to spend time later fussing with it.
make sure not to scratch the paint on the booster face where the MC seats a leak will happen.

put DC111 on the MC grommet bores this will prevent the water from corroding the inlet bores.
Install the MC without the tank so the bolts are easy to tighten, then install the tank.

To get a freshly washed tank
NOTE rinse the tank with brake cleaner,
then wash the tank in simple green and hot water a few times,
use a tooth brush to get the tank clean
rinse with alcohol then drain then use blow drier to evaporate the alcohol.

Fill the tank with ATE fluid,
gold is available.

Open the rear B nut and watch it for bubbles put rags under the MC to catch any fluid,
once the bubbles stop,
then open the front b nut do the same thing.
Once the bubbles stop then have the helper press the pedal and open the tip bleeder,
cycle this a few times .

Then go back to the B nut and have the helper press the pedal then open the rear B nut and close it observe for air bubbles.
once clear move to the front b nut and repeat .
once clear then bleed the tip bleeder.
Once clear of air then move to the ABS unit.

Oopen each B nut one at a time and let it gravity bleed , watch the lines for air ,
Once the lies are clear and all of them have been opened,
then have the helper press the foot pedal while you crack each of the ABS B nuts do this a few times you might find air after the the third of forth press.

Once this is done then bleed the wheels in this order
LF RF LR RR
Make sure you see clear clkean fluid come out.
NOTE dont let the tank get low or air can be brought into the system