89 bonneville SSE.. Problems with ABS brake conversion...

[yogidad]

Doing my best to keep my Bonnie humming.. But, almost three years ago we did a conversion from abs to standard master cylinder /power booster set up.. And from day one I had a real mush feeling to brake pedal. You step on it, yes it stops.. But pedal sits low and by the time you apply them feels like you are almost on the floor, you know?
We did new stuff from O'Reilly auto.. ie Mc and booster. Got a junkyard Mc from 90 caddie I think, for the proportioning valves. Did a custom loop from them to the existing brake line.. Only changes to the likes were at the Mc.. Anyone think they could be my problem? That or the Mc just is not sending enough fluid pressure to the rears due to wrong bore sizes or something?

[RJolly87]

Every conversion I have seen has required an extra brake line to be ran to the back. The system is designed to be connected with diagonal wheels on each output of the MC for safety reasons (redundancy namely). That is where I would start, with plumbing.

When you finally have brake engagement, and you immediately release and reapply the brakes, is the engagement point still the same, or is it almost immediate upon reapplying?

[yogidad]

Hey Jolly ...I don't understand your last sentence of your reply, can you restate it please?

[RJolly87]

Do the brakes come up when you pump them?

[yogidad]

No, brake pedal and feel stay low and spongy...

[RJolly87]

You could try a new MC if you haven't already, but I worry about the way the brake lines are set up.

If the MC wasn't properly bled before installing, and the system wasnt thoroughly bled afterwards, that could also cause the issue. Trapped air can cause these issues too.

[yogidad]

Yes.. I think it was bled properly. The latest mechanic that is going over it did a power brake bleed. He agrees that most likely the brake line set up is at fault. Think he will run a direct line to the back drums and do away with the t set up it has now. Actually has two T set ups.. The original one That splits to the back, which is under the car, and the newer one we put together coming off the two proportional valves...

[yogidad]

I read on other posts that reminded me that the original ABS set up had three brake lines coming from the master cylinder, one for each front wheel (discs) and one to the rear that gets split halfway back under the car, with a line going to each rear wheel. The way my conversion was done, I have two rear lines being split twice, once just inches from the master under the hood, and the other the original set up with the line getting split under the car with one line to each rear wheel. To me, this doesn't sound Kosher...what do you think?

[yogidad]

Actually, the two rear lines aren't split as they come off the MC, but rather are joined together into one with a T , that goes under the car to the rear...

[RJolly87]

Honestly, I don't see how your brake line configuration would cause the symptoms you are having, unless it is messing with the seals in the master cylinder. I would still recommend you run new lines to the back anyway for safety reasons. The system is designed so that if you have catastrophic fluid loss at one brake channel, the other brake channel can still build pressure and stop the car. ABS was designed the same way, only there are 3 channels instead of 2. The way it's configured right now, ANY significant brake pressure loss (think failed caliper, wheel cylinder, or blown line) WILL cause a complete and total failure of the braking system.

As it stands, a low spongy pedal typically indicates that the system is not generating hydraulic pressure, whether it is a lack of pushing fluid (master cylinder), air in the system, or a component not holding pressure, is anyone's guess.

[yogidad]

Ok then. So you are recommending, Mr. Jolly, that I replace the master cylinder, and run new lines to the rear. I can get my guys to do that..let's see

[yogidad]

One other thing, if you might know. Can the brake pedal rod, that connects from the booster to the pedal, could the length of that be changed to move the pedal up a little, so it doesn't start out sitting so low? Besides the soft pedal feel, the pedal may be sitting a bit low. My mechanic friend Lewis that helped me do the conversion from ABS 3 years ago, has always contended when I complain about the pedal sinking almost to the floor when applied ( it stops the car of course, just does not instill confidence to this driver,,) that if we did something to the length of this connecting rod that it would fix the issue. He says that the older ABS unit and the newer non ABS set up are perhaps mechanically different enough to make this an issue?

[RJolly87]

This thread may be worth looking over:
http://www.pontiacbonnevilleclub.com/fo ... 30603.html

There is apparently a slight modification needed at the brake pedal, but I don't know what it is without seeing it.

I would get the lines straightened out first. Also make sure the master cylinder you picked up is actually interchangeable with this car. You mentioned Caddy, which could go either way. If it's not interchangeable, it certainly can't be helping anything. I am pretty sure most units from 88-90 Olds 88, Buick Lesabre, and Pontiac Bonneville should be safe bets if you find out it's not interchangeable.

[yogidad]

sorry..this is a most confusing situation , and explaining it all chronologically has been a pain in the arse too...The master cylinder and power booster on the car presently were purchased new from Oreiily auto parts here in LA, for a non ABS Pontiac Bonneville model...we got the junkyard cadillac master just for the proportional valves that we removed and put on the new master cyl.