Poor brake performance

[Jaumme]

Hello, since i got my 1993 850 t5, the braking performance was pretty low, i thought it was normal untill i drove my friends 1997 V70 LPT on this car the amount of force needed for the same braking power is like 1/3 compared to mine its has another brake circuit with different brake booster but still same calipers and rotors diameter, i did de typical, replaced vacuum hose, brake pads and rotors, bleeded all the old fluid with new one and it remains the same, the calipers all were free moving, the pedal is not low its just quite hard to push, i noticed when bleeding it, even with the caliper bleeder open it was still hard to push, the brake booster seems fine, no leaks or hissing sounds arround it but after the engine is shut off it loses the vacum in arround 1 minute, i think thats the check valve, another thing i noticed is when you are driving and you push hard on the brakes it takes at least 0.5 seconds to the pedal to drop and start braking, and if u are in a downhill and u hold hard the brake pedal when i release it, it takes the same amount of time to the brakes to completely release and let the car roll down. I saw a brake power regulator at the outlet of the master cylinder but i dont know if that can be broken or something else, if you know something similar or something i shoud check plese let me know. Thank you

[scot850]

If the calipers pistons are moving freely and all the slider pins are good, then did you check/replace the flexi-hoses to the calipers? These will collapse internally with age which makes for hard bleeding and poor brake performance.

Does the brake peddle have a hard feel to it or is it very soft? If so it could be the brake booster has a bad check valve in it. One a 93 I can't recall if it is in the booster where the vacuum line goes to it.

Neil.

[Jaumme]

i never changed the flexi-hoses, the pedal is hard, and about the check valve, it is on the brake booster i pulled it with the engine runing and it was sucking air correcly. Im going to buy some flexy hoses, maybe a metal ones and replace them, even if those are not colapsed is not a bad idea to replace them after 26 years

[scot850]

If your peddle is hard when you first start the car after say sitting over night, but then gets softer after a short period it could still be the 1 way valve failing or failed so the booster does not hold vacuum. Changing the hoses are a good idea if they are not original. Some like the metal sleeved hoses, but some say they find the peddle has less feel and is harder. If it improves after you change the hoses using metal hoses it will still feel a lot better than now. I had a 93 850 GLT and it had great brakes.

IF all this does not improve the brake quality then maybe it is the booster itself that is bad.

Neil.

[kallekula]

I have a similar problem with my 2000 S70. Brakes require hard pressure on the pedal. Sitting at a red light the car will eventually start to move.
I bled the brakes, checked booster with vacuum pump for leaks, swapped out the master cylinder but no difference. The last thing I can think about is that there might be a problem with the ABS pump. I have a used new one from a low mileage car but I kind of lost interest. Have two other junk cars that need attention too.

[Jaumme]

so i went ahead and removed the flexi hoses, the first thing i did was to blow through them to see if they were collapsed well they were not, i asked a friend to push the pedal and even doing the maximum force no fluid was comming out the metal line without the hose atached, i disconected the line that goes from the master cylincer to the abs and there the fluid was coming with pressure without force on the pedal, i conected that in its place and disconected the hose that goes from the abs module to de front right wheel (thats the one thatno fluid was comming out) and no fluid is comming though the abs module, in every other line the fluid is comming out correctly, i beliebe a valve from the abs is stuck, i found an abs unit in a nearby junkyard, i will replace that in the next days and will see.

[kallekula]

so i went ahead and removed the flexi hoses, the first thing i did was to blow through them to see if they were collapsed well they were not, i asked a friend to push the pedal and even doing the maximum force no fluid was comming out the metal line without the hose atached, i disconected the line that goes from the master cylincer to the abs and there the fluid was coming with pressure without force on the pedal, i conected that in its place and disconected the hose that goes from the abs module to de front right wheel (thats the one thatno fluid was comming out) and no fluid is comming though the abs module, in every other line the fluid is comming out correctly, i beliebe a valve from the abs is stuck, i found an abs unit in a nearby junkyard, i will replace that in the next days and will see.

Let us now how it went!

[scot850]

I could possibly be the ABS module. There was a thread recently where someone was finding brake fluid in the ABS module housing. Matty Moo at Mid-West ABS said he had seen a few like that, that had been sent in for repair. Maybe just a pure age related issue with the valves either leaking or sticking. Maybe a used ABS module, even if not working correctly might show if that is an issue.

I have a 2000 V70R that can have a hard pedal once in a while, especially when first driven having been sitting, say over night. I suspect the vacuum valve in my booster is the issue. Just a swine to get to on a turbo.

Neil.

[Jaumme]

Yesterday i replaced the ABS module and yes that was the issue now the car brakes much better but the manual says this: If system has been completely or partially drained of brake fluid, system must be pressure-bled. I didnt presure bleed because i dont have a presurre bleeder, i just did the typical bleed with a helper so i dont know if some air can still remain in the system, despite that the car brakes much better than before but the pedal feels softer than my friends v70.

[scot850]

It sounds like you still have air in the system. Pressure bleeding is the only way to get that out. Sounds like you are on the right path though!

Neil.