90 351W EFI Bronoc 185,000 miles

Came home today and 3 blocks from home brakes sink to floor. When i hit them again they are hard but then sink slowly to floor again. I see some wet spot down the front of the booster under the masgter cylinder but the reservoir is still full. Im not sure it didnt looked"wet" before now that i think of it.

I had the rabs proportioing valve replaced when i first got the truck 5 years ago (took a long time for the mechanic to figure that one out) so im assuming its the master cylinder first?

I can pick up a master cylinder at the parts store in the morning from what ive read so far that seems to be the problem.

Just wonder what the best steps to do this are or what should i watch out for.



thanks

 

Oh its most assuredly the MC. Take care to bleed it completely. Air is your worst enemy... it retains moisture and compresses much more than brake fluid which will leave you with a very spongy pedal and/or rust in the system. Be very careful around the booster. The spring inside that lil' bugger can be lethal (I'm serious) if the booster is damaged. Take the time to have a listen to the booster shortly after you shut the engine down. Make sure its not leaking vacuum. DO NOT attempt to open or repair a damaged booster.

 

90 351W EFI
i had problems with one of the line fittings, I used a line wrench and put some liquid wrench on it first too but no luck. It stripped a bit then got worse and not its toast.
Anyway what i ended up doing is unbolted everything else and rotating the master cylinder off the proportioning valve to which to stripped line fitting is connected.

However i realise I cant now bench bleed the master cylinder and have to do it once its attached to the power booster. Is this a problem. Can i get all the air out of the system eventually by just bleeding at the wheels.?

|Are the 2 fittings isolated on the master cylinder? |Can i still bleed one separatly from the other?

The fitting thats stuck to the proportioning valve is the rear one closest to the firewall.

I suppose i could take it to a shop and get the line cut off but if it will work like this id like to try it.

 

The two fittings ARE separate. The front resevoir is for the front axle and the rear for the rear. You can bleed them separately if necessary. What you can do, with the new MC is to buy a short length of brake line with the proper size fittings to fit the MC. Cut it in half. Install the two halves temporarily into the MC. Bend them so that they run back into the resevoirs. Then fill the MC on the truck and bleed it right there. Make sure the ends of the cut brake lines are BELOW the brake fluid surface in the MC. This way you can simply watch and when no more bubbles surface from the ends of the lines, you can remove them and replace the lines to the proportioning valve(s). This way you can bleed the MC on the truck. Trust me, you dont' want to try to bleed all the air from the MC all the way through the system. It will take forever and a LOT of fluid.

As for the trashed fitting, on the end of the stock line, you have one of two options. Cut the line behind the fitting and put a new one on and flare the line. (You could do this with a new piece of line and a compession fitting if you don't have or want to use a flaring tool although the seal is rarely as good at a compression fitting). Or replace the line completely. Either way, the bleeding process on that half of the system is going to take longer.

 

Thanks greystreak. Everythings working great again. For now i rotated the whole master cylinder off proportioning valve that the stripped line fitting is attached too. Then rotated the new(empty) master cylinder onto the valve again,filled it, tightened everything up and bled everything and breaks work great.
I bled the MC, the rear lines and the rabs valvle under the truck.

I cant find any nipples on the front lines at the wheels. How do I bleed these?

Thanks.

 

The bleeder valves are on the top of the calipers on the back side.

 

Thanks,

When i fix the rear fitting on the master cylinder but cutting off the line, ive read on several postings that compression fittings are not a good idea for brake lines. Just wondering thoughts on this? It seems somewhat an easier fix that using a flare tool which ive never done(but willing to if i need be)

 

The best fix is to just replace the rear line to the combo valve(Porportioning valve) from the M/C. It is not hard to do and for thje most part, easy to get to. That way you don't have to worry if you made the correct flare, which by the way, needs to be double flared. You don't want to do half a job on the brake system. if you don't know what you are doing, or if you are not sure, take it to a shop.

You should be able to change out the line easy enough. Then just bleed it at the valve. Gravity bleeding will work for most of the air that will be in the replacement line. Then just bleeding at the valve should do the trick.

Again, let me stress: Brakes are nothing to do half-assed. If you are not comfortable with doing them properly, take it to someone who is.

Good luck,

Dan Harriman 84 F150 lwb
Orange, Texas